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SubjectRe: [RFC PATCH 6/6] x86/jump_label,x86/alternatives: Batch jump label transformations
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On 10/8/18 4:33 PM, Jason Baron wrote:
> On 10/08/2018 08:53 AM, Daniel Bristot de Oliveira wrote:
>> A static key, changing from enabled->disabled/disabled->enabled causes
>> the code to be changed, and this is done in three steps:
>>
>> -- Pseudo-code #1 - Current implementation ---
>> For each key to be updated:
>> 1) add an int3 trap to the address that will be patched
>> sync cores (send IPI to all other CPUs)
>> 2) update all but the first byte of the patched range
>> sync cores (send IPI to all other CPUs)
>> 3) replace the first byte (int3) by the first byte of replacing opcode
>> sync cores (send IPI to all other CPUs)
>> -- Pseudo-code #1 ---
>>
>> The number of IPIs sent is then linear with regard to the number 'n' of
>> entries of a key: O(n*3), which is O(n). For instance, as the static key
>> netstamp_needed_key has four entries (used in for places in the code)
>> in our kernel, 3 IPIs were generated for each entry, resulting in 12 IPIs.
>>
>> This algorithm works fine for the update of a single key. But we think
>> it is possible to optimize the case in which a static key has more than
>> one entry. For instance, the sched_schedstats jump label has 56 entries
>> in my (updated) fedora kernel, resulting in 168 IPIs for each CPU in
>> which the thread that is enabling is _not_ running.
>>
>> In this patch, rather than doing each updated at once, it queue all
>> updates first, and the, apply all updates at once, rewriting the
>> pseudo-code #1 in this way:
>>
>> -- Pseudo-code #2 - This patch ---
>> 1) for each key in the queue:
>> add an int3 trap to the address that will be patched
>> sync cores (send IPI to all other CPUs)
>>
>> 2) for each key in the queue:
>> update all but the first byte of the patched range
>> sync cores (send IPI to all other CPUs)
>>
>> 3) for each key in the queue:
>> replace the first byte (int3) by the first byte of replacing opcode
>>
>> sync cores (send IPI to all other CPUs)
>> -- Pseudo-code #2 - This patch ---
>>
>> Doing the update in this way, the number of IPI becomes O(3) with regard
>> to the number of keys, which is O(1).
>>
>> Currently, the jump label of a static key is transformed via the arch
>> specific function:
>>
>> void arch_jump_label_transform(struct jump_entry *entry,
>> enum jump_label_type type)
>>
>> The new approach (batch mode) uses two arch functions, the first has the
>> same arguments of the arch_jump_label_transform(), and is the function:
>>
>> void arch_jump_label_transform_queue(struct jump_entry *entry,
>> enum jump_label_type type)
>>
>> Rather than transforming the code, it adds the jump_entry in a queue of
>> entries to be updated.
>>
>> After queuing all jump_entries, the function:
>>
>> void arch_jump_label_transform_apply(void)
>>
>> Applies the changes in the queue.
>>
>> The batch of operations was:
>> Suggested-by: Scott Wood <swood@redhat.com>
>
> Hi,
>
> We've discussed a 'batch' mode here before, and we had patches in the
> past iirc, but they never quite reached a merge-able state.

Hi Jason!

Thanks for your comments! I will try to find references for old patches!

I think for
> this patch, we want to separate it in 2 - the text patching code that
> now takes a list, and the jump_label code consumer. Comments below.
>

I see your point. I agree on breaking this patch into two.

>>
>> Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
>> Cc: Masami Hiramatsu <mhiramat@kernel.org>
>> Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>> Cc: Zhou Chengming <zhouchengming1@huawei.com>
>> Cc: Jiri Kosina <jkosina@suse.cz>
>> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
>> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
>> Cc: Chris von Recklinghausen <crecklin@redhat.com>
>> Cc: Jason Baron <jbaron@akamai.com>
>> Cc: Scott Wood <swood@redhat.com>
>> Cc: Marcelo Tosatti <mtosatti@redhat.com>
>> Cc: Clark Williams <williams@redhat.com>
>> Cc: x86@kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>> arch/x86/include/asm/jump_label.h | 2 +
>> arch/x86/include/asm/text-patching.h | 9 +++
>> arch/x86/kernel/alternative.c | 83 +++++++++++++++++++++++++---
>> arch/x86/kernel/jump_label.c | 54 ++++++++++++++++++
>> include/linux/jump_label.h | 5 ++
>> kernel/jump_label.c | 15 +++++
>> 6 files changed, 161 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
>> index 8c0de4282659..d61c476046fe 100644
>> --- a/arch/x86/include/asm/jump_label.h
>> +++ b/arch/x86/include/asm/jump_label.h
>> @@ -15,6 +15,8 @@
>> #error asm/jump_label.h included on a non-jump-label kernel
>> #endif
>>
>> +#define HAVE_JUMP_LABEL_BATCH
>> +
>> #define JUMP_LABEL_NOP_SIZE 5
>>
>> #ifdef CONFIG_X86_64
>> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
>> index e85ff65c43c3..a28230f09d72 100644
>> --- a/arch/x86/include/asm/text-patching.h
>> +++ b/arch/x86/include/asm/text-patching.h
>> @@ -18,6 +18,14 @@ static inline void apply_paravirt(struct paravirt_patch_site *start,
>> #define __parainstructions_end NULL
>> #endif
>>
>> +struct text_to_poke {
>> + struct list_head list;
>> + void *opcode;
>> + void *addr;
>> + void *handler;
>> + size_t len;
>> +};
>> +
>> extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>>
>> /*
>> @@ -37,6 +45,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>> extern void *text_poke(void *addr, const void *opcode, size_t len);
>> extern int poke_int3_handler(struct pt_regs *regs);
>> extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
>> +extern void text_poke_bp_list(struct list_head *entry_list);
>> extern int after_bootmem;
>>
>> #endif /* _ASM_X86_TEXT_PATCHING_H */
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index a4c83cb49cd0..3bd502ea4c53 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -735,9 +735,12 @@ static void do_sync_core(void *info)
>>
>> static bool bp_patching_in_progress;
>> static void *bp_int3_handler, *bp_int3_addr;
>> +struct list_head *bp_list;
>>
>> int poke_int3_handler(struct pt_regs *regs)
>> {
>> + void *ip;
>> + struct text_to_poke *tp;
>> /*
>> * Having observed our INT3 instruction, we now must observe
>> * bp_patching_in_progress.
>> @@ -753,21 +756,38 @@ int poke_int3_handler(struct pt_regs *regs)
>> if (likely(!bp_patching_in_progress))
>> return 0;
>>
>> - if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
>> + if (user_mode(regs))
>> return 0;
>>
>> - /* set up the specified breakpoint handler */
>> - regs->ip = (unsigned long) bp_int3_handler;
>> + /*
>> + * Single poke.
>> + */
>> + if (bp_int3_addr) {
>> + if (regs->ip == (unsigned long) bp_int3_addr) {
>> + regs->ip = (unsigned long) bp_int3_handler;
>> + return 1;
>> + }
>> + return 0;
>> + }
>>
>> - return 1;
>> + /*
>> + * Batch mode.
>> + */
>> + ip = (void *) regs->ip - sizeof(unsigned char);
>> + list_for_each_entry(tp, bp_list, list) {
>> + if (ip == tp->addr) {
>> + /* set up the specified breakpoint handler */
>> + regs->ip = (unsigned long) tp->handler;
>> + return 1;
>> + }
>> + }
>>
>> + return 0;
>> }
>>
>> static void text_poke_bp_set_handler(void *addr, void *handler,
>> unsigned char int3)
>> {
>> - bp_int3_handler = handler;
>> - bp_int3_addr = (u8 *)addr + sizeof(int3);
>> text_poke(addr, &int3, sizeof(int3));
>> }
>>
>> @@ -812,6 +832,9 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>>
>> lockdep_assert_held(&text_mutex);
>>
>> + bp_int3_handler = handler;
>> + bp_int3_addr = (u8 *)addr + sizeof(int3);
>> +
>> bp_patching_in_progress = true;
>> /*
>> * Corresponding read barrier in int3 notifier for making sure the
>> @@ -841,7 +864,53 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>> * the writing of the new instruction.
>> */
>> bp_patching_in_progress = false;
>> -
>> + bp_int3_handler = bp_int3_addr = 0;
>> return addr;
>> }
>>
>> +void text_poke_bp_list(struct list_head *entry_list)
>> +{
>> + unsigned char int3 = 0xcc;
>> + int patched_all_but_first = 0;
>> + struct text_to_poke *tp;
>> +
>> + bp_list = entry_list;
>> + bp_patching_in_progress = true;
>> + /*
>> + * Corresponding read barrier in int3 notifier for making sure the
>> + * in_progress and handler are correctly ordered wrt. patching.
>> + */
>> + smp_wmb();
>> +
>> + list_for_each_entry(tp, entry_list, list)
>> + text_poke_bp_set_handler(tp->addr, tp->handler, int3);
>> +
>> + on_each_cpu(do_sync_core, NULL, 1);
>> +
>> + list_for_each_entry(tp, entry_list, list) {
>> + if (tp->len - sizeof(int3) > 0) {
>> + patch_all_but_first_byte(tp->addr, tp->opcode, tp->len, int3);
>> + patched_all_but_first++;
>> + }
>> + }
>> +
>> + if (patched_all_but_first) {
>> + /*
>> + * According to Intel, this core syncing is very likely
>> + * not necessary and we'd be safe even without it. But
>> + * better safe than sorry (plus there's not only Intel).
>> + */
>> + on_each_cpu(do_sync_core, NULL, 1);
>> + }
>> +
>> + list_for_each_entry(tp, entry_list, list)
>> + patch_first_byte(tp->addr, tp->opcode, int3);
>> +
>> + on_each_cpu(do_sync_core, NULL, 1);
>> + /*
>> + * sync_core() implies an smp_mb() and orders this store against
>> + * the writing of the new instruction.
>> + */
>> + bp_list = 0;
>> + bp_patching_in_progress = false;
>> +}
>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
>> index de588ff47f81..3da5af5de4d3 100644
>> --- a/arch/x86/kernel/jump_label.c
>> +++ b/arch/x86/kernel/jump_label.c
>> @@ -12,6 +12,8 @@
>> #include <linux/list.h>
>> #include <linux/jhash.h>
>> #include <linux/cpu.h>
>> +#include <linux/slab.h>
>> +#include <linux/list.h>
>> #include <asm/kprobes.h>
>> #include <asm/alternative.h>
>> #include <asm/text-patching.h>
>> @@ -139,6 +141,58 @@ void arch_jump_label_transform(struct jump_entry *entry,
>> mutex_unlock(&text_mutex);
>> }
>>
>> +LIST_HEAD(batch_list);
>> +
>> +void arch_jump_label_transform_queue(struct jump_entry *entry,
>> + enum jump_label_type type)
>> +{
>> + struct text_to_poke *tp;
>> +
>> + /*
>> + * Batch mode disabled at boot time.
>> + */
>> + if (early_boot_irqs_disabled)
>> + goto fallback;
>> +
>> + /*
>> + * RFC Note: I put __GFP_NOFAIL, but I could also goto fallback;
>> + * thoughts?
>> + */
>> + tp = kzalloc(sizeof(struct text_to_poke), GFP_KERNEL | __GFP_NOFAIL);
>> + tp->opcode = kzalloc(sizeof(union jump_code_union),
>> + GFP_KERNEL | __GFP_NOFAIL);
>
>
> I wonder if we should just set aside a page here so that we can avoid
> the allocation altogether. I think the size of the text_to_poke on
> x86_64 is 44 bytes, so that's 93 or so entries, which I think covers the
> use-case here. If we go over that limit, we would just do things in
> batches of 93. I just think its nice to avoid memory allocations here to
> avoid creating additional dependencies, although I'm not aware of any
> specific ones.

Yeah, the memory allocation is the "weak" point. In the initial
implementation, I was passing all the arguments from

__jump_label_update()

to the arch code. But I ended up mixing a lot of non-arch with arch
code. It was ugly.

Then, I created the functions in the way that they are now. But I was
putting the entries in a static allocated vector of entries. It worked
fine, but it was not efficient w.r.t memory allocation.

I decided to try using the list with memory allocation because it would
not "wast" memory, and would not require to have a limited amount of
entries (what is better for maintenance...). The bad points about the
memory allocation are:

1) The alloc/list/free costs;
2) What to do when there is no memory.

The 1) is not as bad as it seems, because:

a) it is in the preemptive/irqs enabled context, so it does not cause
latency in the -rt case;
b) it is in the "absolute slow path" - as mentioned in [1];
c) Even doing these operations, this method is faster for the case in
which more than one key is being updated - and these are the performance
sensitive case, thinking "machine wise."

Regarding performance, I tested this patch in the -rt as well, and it
did not cause latency (well, more tests are welcome). Rather, I was
seeing a reduction in the average latency on all CPUs.

Regarding 2) I put the nofail because it looks cleaner. But I also had a
version in which, in the case of a system failing to allocate memory, it
simply falls back to the regular case (a goto fallback) in that
function.

Anyway, I agree that this is the point of doubts about this patch (that
is my opinion as well), and I would like to hear people's opinion about it.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/jump_label.h#n56

>> +
>> + __jump_label_set_jump_code(entry, type, 0, tp->opcode);
>> + tp->addr = (void *) entry->code;
>> + tp->len = JUMP_LABEL_NOP_SIZE;
>> + tp->handler = (void *) entry->code + JUMP_LABEL_NOP_SIZE;
>> +
>> + list_add_tail(&tp->list, &batch_list);
>> +
>> + return;
>> +
>> +fallback:
>> + arch_jump_label_transform(entry, type);
>> +}
>> +
>> +void arch_jump_label_transform_apply(void)
>> +{
>> + struct text_to_poke *tp, *next;
>> +
>> + if (early_boot_irqs_disabled)
>> + return;
>> +
>> + mutex_lock(&text_mutex);
>> + text_poke_bp_list(&batch_list);
>> + mutex_unlock(&text_mutex);
>> +
>> + list_for_each_entry_safe(tp, next, &batch_list, list) {
>> + list_del(&tp->list);
>> + kfree(tp->opcode);
>> + kfree(tp);
>> + }
>> +}
>> +
>> static enum {
>> JL_STATE_START,
>> JL_STATE_NO_UPDATE,
>> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
>> index cd3bed880ca0..2aca92e03494 100644
>> --- a/include/linux/jump_label.h
>> +++ b/include/linux/jump_label.h
>> @@ -156,6 +156,11 @@ extern void jump_label_lock(void);
>> extern void jump_label_unlock(void);
>> extern void arch_jump_label_transform(struct jump_entry *entry,
>> enum jump_label_type type);
>> +#ifdef HAVE_JUMP_LABEL_BATCH
>> +extern void arch_jump_label_transform_queue(struct jump_entry *entry,
>> + enum jump_label_type type);
>> +extern void arch_jump_label_transform_apply(void);
>> +#endif
>> extern void arch_jump_label_transform_static(struct jump_entry *entry,
>> enum jump_label_type type);
>> extern int jump_label_text_reserved(void *start, void *end);
>> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
>> index 940ba7819c87..f534d9c4e07f 100644
>> --- a/kernel/jump_label.c
>> +++ b/kernel/jump_label.c
>> @@ -377,6 +377,7 @@ bool jump_label_can_update_check(struct jump_entry *entry)
>> return 0;
>> }
>>
>> +#ifndef HAVE_JUMP_LABEL_BATCH
>> static void __jump_label_update(struct static_key *key,
>> struct jump_entry *entry,
>> struct jump_entry *stop)
>> @@ -386,6 +387,20 @@ static void __jump_label_update(struct static_key *key,
>> arch_jump_label_transform(entry, jump_label_type(entry));
>> }
>> }
>> +#else
>> +static void __jump_label_update(struct static_key *key,
>> + struct jump_entry *entry,
>> + struct jump_entry *stop)
>> +{
>> + for_each_label_entry(key, entry, stop) {
>> + if (jump_label_can_update_check(entry))
>> + arch_jump_label_transform_queue(entry,
>> + jump_label_type(entry));
>> + }
>
> So this could be done in batches if there are more entries than
> PAGE_SIZE / sizeof(struct text_to_poke)

Yeah, but that was one of the reasons for me to try the alloc/list. As
this is the slow path, maintenance and clarity of the code is "a point."

Anyway, I am not against doing the static allocation! Rather, if people
agree this is the way to go, I will go for it as well.

-- Daniel

>
>> + arch_jump_label_transform_apply();
>> +
>> +}
>> +#endif
>>
>> void __init jump_label_init(void)
>> {
>>

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