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SubjectRe: ARC Assembler: bundle_align_mode directive support
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On 6/11/19 11:47 AM, Eugeniy Paltsev wrote:
> Hi Vineet,
>
> On Mon, 2019-06-10 at 15:55 +0000, Vineet Gupta wrote:
>> On 6/8/19 11:21 AM, Eugeniy Paltsev wrote:
>>> Hi Cupertino,
>>>
>>> I tried to use ".bundle_align_mode" directive in ARC assembly, but I got following error:
>>> ----------------->8--------------
>>> Assembler messages:
>>> Error: unknown pseudo-op: `.bundle_align_mode'
>>> ----------------->8--------------
>>>
>>> Is it possible to implement it in ARC assembler?
>>> There is some context about the reason we want to have it:
>>>
>>> I'm trying to add support of jump labels for ARC in linux kernel. Jump labels
>>> provide an interface to generate static branches using self-modifying code.
>>> This allows us to implement conditional branches where changing branch
>>> direction is expensive but branch selection is basically 'free'.
>>>
>>> There is nuance in current implementation:
>>> We need to patch code by rewriting 32-bit NOP by 32-bit BRANCH instruction (or vice versa).
>>> It can be easily done with following code:
>>> ----------------->8--------------
>>> write_32_bit(new_instruction)
>>> flush_l1_dcache_range_this_cpu
>>> invalidate_l1_icache_range_all_cpu
>>> ----------------->8--------------
>>>
>>> I$ update will be atomic in most of cases except the patched instruction share
>>> two L1 cache lines (so first 16 bits of instruction are in the one cache line and
>>> last 16 bit are in another cache line).
>>> In such case we can execute half-updated instruction if we are patching live code (and we are unlucky enough :)
>>
>> While I understand your need for alignment, I don't see how you can possibly
>> execute stray lines.
>> dcache flush will be propagated by hardware (SCU) to all cores (as applicable) and
>> the icache cache flush xcall is synchronous and will have to finish on all cores
>> before we proceed to execute the cod eitself.
>>
>
> It's easy as hell - we may execute some code on one CPU and patch it on another CPU at the same moment :)
>
> And here is the example of the situation when we can get the issue:
> - Let's imagine we have SMP system with CPU#0 and CPU#1.
> - We have instruction which share two L1 cache lines:
> ~ ---------------------------------|--------------------------------- ~
> ~ |instruction-Z (32-bit instruction we patch)| ~
> ~ ---------------------------------|--------------------------------- ~
> ~ cache-line-X | cache-line-Y ~
> ~ ---------------------------------|--------------------------------- ~
>
> CPU#0 is trying to switch our static branch, so it will patch the code (instruction-Z)
> CPU#1 is executing this code with our static branch, so it execute the code (with instruction-Z) that will be patched.
>
> 1) CPU#0: we generate new instruction (to replace 'instruction-Z')
> and write it with 32-bit store (st). It is written to CPU#0 L1 data cache.
> 2) CPU#0: we call our function flush_icache_range.
> It firstly will flush L1 data cache region on CPU#0.
> In our example it will flush CPU#0 L1 'cache-line-X' and 'cache-line-Y' to L2 cache.
> 3) CPU#1: we are executing code which is in 'cache-line-X'.
> 'cache-line-Y' is __NOT__ in the L1 instruction cache of CPU#1.
> We need to execute 'instruction-Z', but only half of the instruction is in L1 instruction cache.
> CPU#1 fetch 'cache-line-Y' to L1 instruction cache.
> 4) Ooops: now we have corrupted 'instruction-Z' in L1 instruction cache of CPU#1:
> First 16 bit of this instruction (which belongs to 'cache-line-X') are old (not patched).
> Last 16 bit of this instruction (which belongs to 'cache-line-Y') are new (patched).

OK I agree there is a race: I was not thinking of case where the exact patched
instruction is concurrently being executed on other core. Indeed we need to ensure
it doesn't straddle icache line.

We could potentially avoid all of these issues if we could use 2 byte (NOP_S +
B_S). The added advantage is even better icache footprint. Ofcourse with B_S the
branch range goes down from S25 to S10, but considering the use cases it might be
enough after all.

-------->8-----------
static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
asm_volatile_goto("1:\n\t"
- "nop \n\t"
+ "nop_s \n\t"
".pushsection __jump_table, \"aw\"\n\t"
".word 1b, %l[l_yes], %c0\n\t"
".popsection\n\t"

static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
branch)
{
asm_volatile_goto("1:\n\t"
- "b %l[l_yes]\n\t"
+ "b_s %l[l_yes]\n\t"
-------->8-----------

It indeed works, except for 1 place in networking code where S10 is not enough.
Granted it is not future proof, I do like the effect

add/remove: 0/0 grow/shrink: 0/45 up/down: 0/-244 (-244)
function old new delta
udp_queue_rcv_one_skb 1010 1006 -4
udp_flow_hashrnd 80 76 -4
udp_destroy_sock 128 124 -4
udp4_lib_lookup2.constprop 562 558 -4
udp4_gro_receive 628 624 -4
try_to_wake_up 876 872 -4
tcp_splice_read 1026 1022 -4
tcp_recvmsg 2618 2614 -4
tcp_read_sock 508 504 -4
switched_from_fair 176 172 -4
secure_ipv4_port_ephemeral 102 98 -4
sched_clock_cpu 16 12 -4
run_filter 232 228 -4
process_backlog 420 416 -4
pick_next_task_fair 2984 2980 -4
netif_rx_internal 174 170 -4
netif_reset_xps_queues 690 686 -4
netif_receive_skb_list 488 484 -4
netif_receive_skb_internal 242 238 -4
load_balance 2370 2366 -4
inet_sendpage 352 348 -4
inet_sendmsg 244 240 -4
inet_recvmsg 192 188 -4
inet_lhash2_lookup 414 410 -4
inet_accept 252 248 -4
housekeeping_affine 30 26 -4
fnhe_hashfun 158 154 -4
finish_task_switch 370 366 -4
enqueue_task_fair 1504 1500 -4
do_page_fault 658 654 -4
dequeue_task_fair 988 984 -4
bpf_user_rnd_init_once 62 58 -4
__skb_get_hash_symmetric 694 690 -4
__skb_get_hash 726 722 -4
__schedule 998 994 -4
__release_sock 240 236 -4
__netif_receive_skb_core 1782 1778 -4
__netdev_alloc_skb 304 300 -4
task_tick_fair 814 806 -8
sched_rt_period_timer 1034 1026 -8
netdev_pick_tx 664 656 -8
find_busiest_group 2348 2340 -8
check_preempt_wakeup 690 682 -8
jump_label_test 324 312 -12
select_task_rq_fair 3112 3072 -40
Total: Before=4064620, After=4064376, chg -1.000000%



>>> As of today I simply align by 4 byte instruction which can be patched with ".balign 4" directive:
>>> However 'align by 4' directive is much stricter than it actually required.
...

> All instructions on ARCv2 are aligned by 2 byte (even if the instruction is 4-byte long).
>
> Using '.balign' directive we align instruction which can be
> patched by 4 byte.
> So compiler will add one 'nop_s' before our instruction if it is not 4 byte aligned.
>

...

>
> In most of the cases this extra 'nop_s' is unnecessary as it is fine to have our instruction
> not 4 byte aligned if it doesn't cross l1 cache line boundary.
>
> It is't the huge problem - but we are adding one unnecessary 'nop' while we try to optimize hot code.
>
...

> If we will have ".bundle_align_mode" directive supported we can add extra 'nop_s' only if it's really required
> (if our instruction _really_ cross L1 cache line boundary). So we won't add useless NOP to hot code.

OK understand, but this is not gating the feature. We can start off with .balign 4
and if-n-once tools support it we can remove the alignment requirements.

-Vineet

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