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SubjectRe: [PATCH v2 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state
On Fri, Apr 12, 2019 at 06:14:20PM +0100, Julien Grall wrote:
> When the kernel is compiled with CONFIG_KERNEL_MODE_NEON, some part of
> the kernel may be able to use FPSIMD/SVE. This is for instance the case
> for crypto code.
>
> Any use of FPSIMD/SVE in the kernel are clearly marked by using the
> function kernel_neon_{begin, end}. Furthermore, this can only be used
> when may_use_simd() returns true.
>
> The current implementation of may_use_simd() allows softirq to use
> FPSIMD/SVE unless it is currently in used (i.e kernel_neon_busy is true).

Nit: "in used" -> "in use"

> When in used, softirqs usually fallback to a software method.

Likewise.

Nit: "fallback" -> "fall back"

> At the moment, as a softirq may use FPSIMD/SVE, softirqs are disabled
> when touching the FPSIMD/SVE context. This has the drawback to disable
> all softirqs even if they are not using FPSIMD/SVE.
>
> As a softirq should not rely on been able to use simd at a given time,
> there are limited reason to keep softirq disabled when touching the

The implication is not totally clear to me here. Maybe write something
like

"Since a softirq is supposed to check may_use_simd() anyway before
attempting to use FPSIMD/SVE, there is limited reason to keep softirq
disabled when touching the FPSIMD/SVE context [...]"

> FPSIMD/SVE context. Instead, we can only disable preemption and tell

I'd put "just" or "simply" instead of "only" here.

> the NEON unit is currently in use.

Maybe "mark the FPSIMD/SVE context as in use by setting the
CPU's kernel_neon_busy flag".

> This patch introduces two new helpers {get, put}_cpu_fpsimd_context to
> mark the area using FPSIMD/SVE context and use them in replacement of

uses

> local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are
> also re-implemented to use the new helpers.
>
> Additionally, this patch introduced a double-underscored version of each

introduces

> helper that can be used when preemption is disabled. This avoid to
> disable/enable preemption for again and helps documenting places where

The wording seems a bit mangled here? Also, these are not for general
use, so maybe say something like

"For use in the fpsimd_thread_switch(), which is a critical path where
preemption is already disabled, double-underscored versions of the
helpers are provided to avoid disabling preemption again."

(I'm assuming here that we don't need to use these elsewhere -- see
other comments.)

> context can only be used by one instance.
>
> This patch has been benchmarked on Linux 5.1-rc4 with defconfig.
>
> On Juno2:
> * hackbench 100 process 1000 (10 times)
> * .7% quicker
>
> On ThunderX 2:
> * hackbench 1000 process 1000 (20 times)
> * 3.4% quicker
>
> Signed-off-by: Julien Grall <julien.grall@arm.com>
>
> ---
> Changes in v2:
> - Remove spurious call to kernel_neon_enable in kernel_neon_begin.
> - Rename kernel_neon_{enable, disable} to {get, put}_cpu_fpsimd_context
> - Introduce a double-underscore version of the helpers for case
> where preemption is already disabled
> - Introduce have_cpu_fpsimd_context() and use it in WARN_ON(...)
> - Surround more places in the code with the new helpers
> - Rework the comments
> - Update the commit message with the benchmark result
> ---
> arch/arm64/include/asm/simd.h | 4 +-
> arch/arm64/kernel/fpsimd.c | 133 ++++++++++++++++++++++++++++++------------
> 2 files changed, 97 insertions(+), 40 deletions(-)
>
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index 6495cc51246f..94c0dac508aa 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -15,10 +15,10 @@
> #include <linux/preempt.h>
> #include <linux/types.h>
>
> -#ifdef CONFIG_KERNEL_MODE_NEON
> -
> DECLARE_PER_CPU(bool, kernel_neon_busy);
>
> +#ifdef CONFIG_KERNEL_MODE_NEON
> +
> /*
> * may_use_simd - whether it is allowable at this time to issue SIMD
> * instructions or access the SIMD register file
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 9e4e4b6acd93..761d848fb26d 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -92,7 +92,8 @@
> * To prevent this from racing with the manipulation of the task's FPSIMD state
> * from task context and thereby corrupting the state, it is necessary to
> * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE
> - * flag with local_bh_disable() unless softirqs are already masked.
> + * flag with kernel_neon_{disable, enable}. This will still allow softirqs to

These names don't match the code now.

> + * run but prevent them to use FPSIMD.
> *
> * For a certain task, the sequence may look something like this:
> * - the task gets scheduled in; if both the task's fpsimd_cpu field
> @@ -150,6 +151,58 @@ extern void __percpu *efi_sve_state;
>
> #endif /* ! CONFIG_ARM64_SVE */
>
> +DEFINE_PER_CPU(bool, kernel_neon_busy);
> +EXPORT_PER_CPU_SYMBOL(kernel_neon_busy);

This feels mis-named now. Maybe "fpsimd_context_busy" would be a better
name?

> +
> +/*
> + * Obtain the CPU FPSIMD context for use by the calling context.
> + *
> + * The caller may freely modify FPSIMD context until *put_cpu_fpsimd_context()

Nit: Why *? This makes it look a bit like get_cpu_fpsimd_context()
returns a pointer and you're saying something about dereferencing that
pointer here.

> + * is called.
> + *
> + * The double-underscore version must only be called if you know the task
> + * can't be preempted.
> + *
> + * __get_cpu_fpsimd_context() *must* be in pair with __put_cpu_fpsimd_context()
> + * get_cpu_fpsimd_context() *must* be in pair with put_cpu_fpsimd_context()

"in pair" -> "paired with"?

I'd move each of these comments to be next to the function it applies
to.

> + */
> +static void __get_cpu_fpsimd_context(void)
> +{
> + bool busy = __this_cpu_xchg(kernel_neon_busy, true);
> +

I don't mind whether there is a blank line here or not, but please make
it consistent with __put_cpu_fpsimd_context().

> + WARN_ON(busy);
> +}
> +
> +static void get_cpu_fpsimd_context(void)
> +{
> + preempt_disable();
> + __get_cpu_fpsimd_context();
> +}
> +
> +/*
> + * Release the CPU FPSIMD context.
> + *
> + * Must be called from a context in which *get_cpu_fpsimd_context() was

Nit: Why *?

> + * previously called, with no call to *put_cpu_fpsimd_context() in the
> + * meantime.
> + */
> +static void __put_cpu_fpsimd_context(void)
> +{
> + bool busy = __this_cpu_xchg(kernel_neon_busy, false);
> + WARN_ON(!busy); /* No matching get_cpu_fpsimd_context()? */
> +}
> +
> +static void put_cpu_fpsimd_context(void)
> +{
> + __put_cpu_fpsimd_context();
> + preempt_enable();
> +}
> +
> +static bool have_cpu_fpsimd_context(void)
> +{
> + return (!preemptible() && __this_cpu_read(kernel_neon_busy));

Nit: Redundant ()

> +}
> +
> /*
> * Call __sve_free() directly only if you know task can't be scheduled
> * or preempted.
> @@ -221,11 +274,12 @@ static void sve_free(struct task_struct *task)
> * thread_struct is known to be up to date, when preparing to enter
> * userspace.
> *
> - * Softirqs (and preemption) must be disabled.
> + * The FPSIMD context must be acquired with get_cpu_fpsimd_context()

or __get_cpu_fpsimd_context()? Since this is effectively documented by
the WARN_ON() and this is a local function anyway, maybe it would be
simpler just to drop this comment here?

> + * before calling this function.
> */
> static void task_fpsimd_load(void)
> {
> - WARN_ON(!in_softirq() && !irqs_disabled());
> + WARN_ON(!have_cpu_fpsimd_context());
>
> if (system_supports_sve() && test_thread_flag(TIF_SVE))
> sve_load_state(sve_pffr(&current->thread),
> @@ -239,15 +293,22 @@ static void task_fpsimd_load(void)
> * Ensure FPSIMD/SVE storage in memory for the loaded context is up to
> * date with respect to the CPU registers.
> *
> - * Softirqs (and preemption) must be disabled.
> + * The FPSIMD context must be acquired with get_cpu_fpsimd_context()

Likewise.

> + * before calling this function.
> */
> static void fpsimd_save(void)
> {
> struct fpsimd_last_state_struct const *last =
> this_cpu_ptr(&fpsimd_last_state);
> /* set by fpsimd_bind_task_to_cpu() or fpsimd_bind_state_to_cpu() */
> + WARN_ON(!have_cpu_fpsimd_context());
>
> - WARN_ON(!in_softirq() && !irqs_disabled());
> + if ( !have_cpu_fpsimd_context() )

Nit: Redundant whitespace around expression.

> + {
> + printk("preemptible() = %u kernel_neon_busy = %u\n",
> + preemptible(), __this_cpu_read(kernel_neon_busy));
> + while (1);
> + }
>
> if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
> if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
> @@ -352,7 +413,8 @@ static int __init sve_sysctl_init(void) { return 0; }
> * task->thread.sve_state.
> *
> * Task can be a non-runnable task, or current. In the latter case,
> - * softirqs (and preemption) must be disabled.
> + * the FPSIMD context must be acquired with get_fpu_fpsimd_context()
> + * before calling this function.
> * task->thread.sve_state must point to at least sve_state_size(task)
> * bytes of allocated kernel memory.
> * task->thread.uw.fpsimd_state must be up to date before calling this
> @@ -379,7 +441,8 @@ static void fpsimd_to_sve(struct task_struct *task)
> * task->thread.uw.fpsimd_state.
> *
> * Task can be a non-runnable task, or current. In the latter case,
> - * softirqs (and preemption) must be disabled.
> + * the FPSIMD context must be acquired with get_fpu_fpsimd_context()
> + * before calling this function.
> * task->thread.sve_state must point to at least sve_state_size(task)
> * bytes of allocated kernel memory.
> * task->thread.sve_state must be up to date before calling this function.
> @@ -539,7 +602,7 @@ int sve_set_vector_length(struct task_struct *task,
> * non-SVE thread.
> */
> if (task == current) {
> - local_bh_disable();
> + get_cpu_fpsimd_context();
>
> fpsimd_save();
> }
> @@ -549,7 +612,7 @@ int sve_set_vector_length(struct task_struct *task,
> sve_to_fpsimd(task);
>
> if (task == current)
> - local_bh_enable();
> + put_cpu_fpsimd_context();
>
> /*
> * Force reallocation of task SVE state to the correct size
> @@ -862,7 +925,7 @@ asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs)
>
> sve_alloc(current);
>
> - local_bh_disable();
> + get_cpu_fpsimd_context();
>
> fpsimd_save();
>
> @@ -873,7 +936,7 @@ asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs)
> if (test_and_set_thread_flag(TIF_SVE))
> WARN_ON(1); /* SVE access shouldn't have trapped */
>
> - local_bh_enable();
> + put_cpu_fpsimd_context();
> }
>
> /*
> @@ -917,6 +980,8 @@ void fpsimd_thread_switch(struct task_struct *next)
> if (!system_supports_fpsimd())
> return;
>
> + __get_cpu_fpsimd_context();
> +
> /* Save unsaved fpsimd state, if any: */
> fpsimd_save();
>
> @@ -931,6 +996,8 @@ void fpsimd_thread_switch(struct task_struct *next)
>
> update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
> wrong_task || wrong_cpu);
> +
> + __put_cpu_fpsimd_context();
> }
>
> void fpsimd_flush_thread(void)
> @@ -940,7 +1007,7 @@ void fpsimd_flush_thread(void)
> if (!system_supports_fpsimd())
> return;
>
> - local_bh_disable();
> + get_cpu_fpsimd_context();
>
> fpsimd_flush_task_state(current);
> memset(&current->thread.uw.fpsimd_state, 0,
> @@ -981,7 +1048,7 @@ void fpsimd_flush_thread(void)
> current->thread.sve_vl_onexec = 0;
> }
>
> - local_bh_enable();
> + put_cpu_fpsimd_context();
> }
>
> /*
> @@ -993,9 +1060,9 @@ void fpsimd_preserve_current_state(void)
> if (!system_supports_fpsimd())
> return;
>
> - local_bh_disable();
> + get_cpu_fpsimd_context();
> fpsimd_save();
> - local_bh_enable();
> + put_cpu_fpsimd_context();
> }
>
> /*
> @@ -1012,7 +1079,8 @@ void fpsimd_signal_preserve_current_state(void)
>
> /*
> * Associate current's FPSIMD context with this cpu
> - * Preemption must be disabled when calling this function.
> + * The FPSIMD context should be acquired with get_cpu_fpsimd_context()
> + * before calling this function.
> */
> void fpsimd_bind_task_to_cpu(void)
> {
> @@ -1058,14 +1126,14 @@ void fpsimd_restore_current_state(void)
> if (!system_supports_fpsimd())
> return;
>
> - local_bh_disable();
> + get_cpu_fpsimd_context();
>
> if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
> task_fpsimd_load();
> fpsimd_bind_task_to_cpu();
> }
>
> - local_bh_enable();
> + put_cpu_fpsimd_context();
> }
>
> /*
> @@ -1078,7 +1146,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
> if (!system_supports_fpsimd())
> return;
>
> - local_bh_disable();
> + get_cpu_fpsimd_context();
>
> current->thread.uw.fpsimd_state = *state;
> if (system_supports_sve() && test_thread_flag(TIF_SVE))
> @@ -1089,7 +1157,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
>
> clear_thread_flag(TIF_FOREIGN_FPSTATE);
>
> - local_bh_enable();
> + put_cpu_fpsimd_context();
> }
>
> /*
> @@ -1115,7 +1183,8 @@ void fpsimd_flush_task_state(struct task_struct *t)
>
> /*
> * Invalidate any task's FPSIMD state that is present on this cpu.
> - * This function must be called with softirqs disabled.
> + * The FPSIMD context should be acquired with get_cpu_fpsimd_context()
> + * before calling this function.
> */
> static void fpsimd_flush_cpu_state(void)
> {
> @@ -1125,19 +1194,18 @@ static void fpsimd_flush_cpu_state(void)
>
> /*
> * Save the FPSIMD state to memory and invalidate cpu view.
> - * This function must be called with softirqs (and preemption) disabled.
> + * This function must be called with preemption disabled.
> */
> void fpsimd_save_and_flush_cpu_state(void)
> {
> + __get_cpu_fpsimd_context();
> fpsimd_save();
> fpsimd_flush_cpu_state();
> + __put_cpu_fpsimd_context();

It may be cleaner to avoid the assumption about preemption already being
disabled here. fpsimd_thread_switch() is rather a special case, but for
this one is this really used on a hot path that justifies the assumption?

If not, we could just move to the regular (non-__) functions here and
drop that comment.

[...]

Cheers
---Dave

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