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Subject[PATCH v3 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state
Date
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 use (i.e kernel_neon_busy is true).
When in use, softirqs usually fall back to a software method.

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.

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. Instead, we can simply
disable preemption and mark the FPSIMD/SVE context as in use by setting
CPU's kernel_neon_busy flag.

Two new helpers {get, put}_cpu_fpsimd_context is introduced to mark the
area using FPSIMD/SVE context and uses them in replacement of
local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are
also re-implemented to use the new helpers.

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

The change 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 v3:
- Fix typoes in the commit message
- Rework a bit the commit message
- Use imperative mood
- Rename kernel_neon_busy to fpsimd_context_busy
- Remove debug code
- Update comments
- Don't require preemption when calling fpsimd_save_and_flush_cpu_state()

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 | 10 ++--
arch/arm64/kernel/fpsimd.c | 126 ++++++++++++++++++++++++++++--------------
2 files changed, 88 insertions(+), 48 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index 6495cc51246f..a6307e43b8c2 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -15,9 +15,9 @@
#include <linux/preempt.h>
#include <linux/types.h>

-#ifdef CONFIG_KERNEL_MODE_NEON
+DECLARE_PER_CPU(bool, fpsimd_context_busy);

-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
@@ -29,15 +29,15 @@ DECLARE_PER_CPU(bool, kernel_neon_busy);
static __must_check inline bool may_use_simd(void)
{
/*
- * kernel_neon_busy is only set while preemption is disabled,
+ * fpsimd_context_busy is only set while preemption is disabled,
* and is clear whenever preemption is enabled. Since
- * this_cpu_read() is atomic w.r.t. preemption, kernel_neon_busy
+ * this_cpu_read() is atomic w.r.t. preemption, fpsimd_context_busy
* cannot change under our feet -- if it's set we cannot be
* migrated, and if it's clear we cannot be migrated to a CPU
* where it is set.
*/
return !in_irq() && !irqs_disabled() && !in_nmi() &&
- !this_cpu_read(kernel_neon_busy);
+ !this_cpu_read(fpsimd_context_busy);
}

#else /* ! CONFIG_KERNEL_MODE_NEON */
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 5313aa257be6..6168d06bbd20 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 {, __}get_cpu_fpsimd_context(). This will still allow softirqs to
+ * 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
@@ -155,6 +156,56 @@ extern void __percpu *efi_sve_state;

#endif /* ! CONFIG_ARM64_SVE */

+DEFINE_PER_CPU(bool, fpsimd_context_busy);
+EXPORT_PER_CPU_SYMBOL(fpsimd_context_busy);
+
+static void __get_cpu_fpsimd_context(void)
+{
+ bool busy = __this_cpu_xchg(fpsimd_context_busy, true);
+
+ WARN_ON(busy);
+}
+
+/*
+ * Claim ownership of the CPU FPSIMD context for use by the calling context.
+ *
+ * The caller may freely modify FPSIMD context until *put_cpu_fpsimd_context()
+ * is called.
+ *
+ * The double-underscore version must only be called if you know the task
+ * can't be preempted.
+ */
+static void get_cpu_fpsimd_context(void)
+{
+ preempt_disable();
+ __get_cpu_fpsimd_context();
+}
+
+static void __put_cpu_fpsimd_context(void)
+{
+ bool busy = __this_cpu_xchg(fpsimd_context_busy, false);
+
+ WARN_ON(!busy); /* No matching get_cpu_fpsimd_context()? */
+}
+
+/*
+ * Release the CPU FPSIMD context.
+ *
+ * Must be called from a context in which get_cpu_fpsimd_context() was
+ * previously called, with no call to put_cpu_fpsimd_context() in the
+ * meantime.
+ */
+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(fpsimd_context_busy);
+}
+
/*
* Call __sve_free() directly only if you know task can't be scheduled
* or preempted.
@@ -225,12 +276,10 @@ static void sve_free(struct task_struct *task)
* This function should be called only when the FPSIMD/SVE state in
* thread_struct is known to be up to date, when preparing to enter
* userspace.
- *
- * Softirqs (and preemption) must be disabled.
*/
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),
@@ -243,8 +292,6 @@ 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.
*/
static void fpsimd_save(void)
{
@@ -252,7 +299,7 @@ static void fpsimd_save(void)
this_cpu_ptr(&fpsimd_last_state);
/* set by fpsimd_bind_task_to_cpu() or fpsimd_bind_state_to_cpu() */

- WARN_ON(!in_softirq() && !irqs_disabled());
+ WARN_ON(!have_cpu_fpsimd_context());

if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
@@ -357,7 +404,8 @@ 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 caller must have ownership of the cpu 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
@@ -384,7 +432,8 @@ 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 caller must have ownership of the cpu 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.
@@ -544,7 +593,7 @@ int sve_set_vector_length(struct task_struct *task,
* non-SVE thread.
*/
if (task == current) {
- local_bh_disable();
+ get_cpu_fpsimd_context();

fpsimd_save();
}
@@ -554,7 +603,7 @@ 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
@@ -867,7 +916,7 @@ asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs)

sve_alloc(current);

- local_bh_disable();
+ get_cpu_fpsimd_context();

fpsimd_save();

@@ -878,7 +927,7 @@ 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();
}

/*
@@ -922,6 +971,8 @@ 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();

@@ -936,6 +987,8 @@ 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)
@@ -945,7 +998,7 @@ 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,
@@ -986,7 +1039,7 @@ void fpsimd_flush_thread(void)
current->thread.sve_vl_onexec = 0;
}

- local_bh_enable();
+ put_cpu_fpsimd_context();
}

/*
@@ -998,9 +1051,9 @@ 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();
}

/*
@@ -1017,7 +1070,8 @@ void fpsimd_signal_preserve_current_state(void)

/*
* Associate current's FPSIMD context with this cpu
- * Preemption must be disabled when calling this function.
+ * The caller must have ownership of the cpu FPSIMD context before calling
+ * this function.
*/
void fpsimd_bind_task_to_cpu(void)
{
@@ -1063,14 +1117,14 @@ 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();
}

/*
@@ -1083,7 +1137,7 @@ 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))
@@ -1094,7 +1148,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)

clear_thread_flag(TIF_FOREIGN_FPSTATE);

- local_bh_enable();
+ put_cpu_fpsimd_context();
}

/*
@@ -1120,7 +1174,8 @@ 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)
{
@@ -1128,21 +1183,17 @@ static void fpsimd_flush_cpu_state(void)
set_thread_flag(TIF_FOREIGN_FPSTATE);
}

-/*
- * Save the FPSIMD state to memory and invalidate cpu view.
- * This function must be called with softirqs (and preemption) disabled.
- */
+/* Save the FPSIMD state to memory and invalidate cpu view. */
void fpsimd_save_and_flush_cpu_state(void)
{
+ get_cpu_fpsimd_context();
fpsimd_save();
fpsimd_flush_cpu_state();
+ put_cpu_fpsimd_context();
}

#ifdef CONFIG_KERNEL_MODE_NEON

-DEFINE_PER_CPU(bool, kernel_neon_busy);
-EXPORT_PER_CPU_SYMBOL(kernel_neon_busy);
-
/*
* Kernel-side NEON support functions
*/
@@ -1167,19 +1218,13 @@ void kernel_neon_begin(void)

BUG_ON(!may_use_simd());

- local_bh_disable();
-
- __this_cpu_write(kernel_neon_busy, true);
+ get_cpu_fpsimd_context();

/* Save unsaved fpsimd state, if any: */
fpsimd_save();

/* Invalidate any task state remaining in the fpsimd regs: */
fpsimd_flush_cpu_state();
-
- preempt_disable();
-
- local_bh_enable();
}
EXPORT_SYMBOL(kernel_neon_begin);

@@ -1194,15 +1239,10 @@ EXPORT_SYMBOL(kernel_neon_begin);
*/
void kernel_neon_end(void)
{
- bool busy;
-
if (!system_supports_fpsimd())
return;

- busy = __this_cpu_xchg(kernel_neon_busy, false);
- WARN_ON(!busy); /* No matching kernel_neon_begin()? */
-
- preempt_enable();
+ put_cpu_fpsimd_context();
}
EXPORT_SYMBOL(kernel_neon_end);

--
2.11.0
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