@@ -164,6 +164,7 @@ struct thread_struct {
unsigned int fpsimd_cpu;
void *sve_state; /* SVE registers, if any */
void *sme_state; /* ZA and ZT state, if any */
+ unsigned long sve_timeout; /* jiffies to drop TIF_SVE */
unsigned int vl[ARM64_VEC_MAX]; /* vector length */
unsigned int vl_onexec[ARM64_VEC_MAX]; /* vl after next exec */
unsigned long fault_address; /* fault info */
@@ -354,6 +354,7 @@ static void task_fpsimd_load(void)
{
bool restore_sve_regs = false;
bool restore_ffr;
+ unsigned long sve_vq_minus_one;
WARN_ON(!system_supports_fpsimd());
WARN_ON(preemptible());
@@ -365,18 +366,12 @@ static void task_fpsimd_load(void)
if (system_supports_sve() || system_supports_sme()) {
switch (current->thread.fp_type) {
case FP_STATE_FPSIMD:
- /* Stop tracking SVE for this task until next use. */
- if (test_and_clear_thread_flag(TIF_SVE))
- sve_user_disable();
break;
case FP_STATE_SVE:
if (!thread_sm_enabled(¤t->thread) &&
!WARN_ON_ONCE(!test_and_set_thread_flag(TIF_SVE)))
sve_user_enable();
- if (test_thread_flag(TIF_SVE))
- sve_set_vq(sve_vq_from_vl(task_get_sve_vl(current)) - 1);
-
restore_sve_regs = true;
restore_ffr = true;
break;
@@ -395,6 +390,15 @@ static void task_fpsimd_load(void)
}
}
+ /*
+ * If SVE has been enabled we may keep it enabled even if
+ * loading only FPSIMD state, so always set the VL.
+ */
+ if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
+ sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1;
+ sve_set_vq(sve_vq_minus_one);
+ }
+
/* Restore SME, override SVE register configuration if needed */
if (system_supports_sme()) {
unsigned long sme_vl = task_get_sme_vl(current);
@@ -421,6 +425,25 @@ static void task_fpsimd_load(void)
} else {
WARN_ON_ONCE(current->thread.fp_type != FP_STATE_FPSIMD);
fpsimd_load_state(¤t->thread.uw.fpsimd_state);
+
+ /*
+ * If the task had been using SVE we keep it enabled
+ * when loading FPSIMD only state for a period to
+ * minimise overhead for tasks actively using SVE,
+ * disabling it periodicaly to ensure that tasks that
+ * use SVE intermittently do eventually avoid the
+ * overhead of carrying SVE state. The timeout is
+ * initialised when we take a SVE trap in in
+ * do_sve_acc().
+ */
+ if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
+ if (time_after(jiffies, current->thread.sve_timeout)) {
+ clear_thread_flag(TIF_SVE);
+ sve_user_disable();
+ } else {
+ sve_flush_live(true, sve_vq_minus_one);
+ }
+ }
}
}
@@ -1397,6 +1420,13 @@ void do_sve_acc(unsigned long esr, struct pt_regs *regs)
get_cpu_fpsimd_context();
+ /*
+ * We will keep SVE enabled when loading FPSIMD only state for
+ * the next second to minimise traps when userspace is
+ * actively using SVE.
+ */
+ current->thread.sve_timeout = jiffies + HZ;
+
if (test_and_set_thread_flag(TIF_SVE))
WARN_ON(1); /* SVE access shouldn't have trapped */
When we are in a syscall we take the opportunity to discard the SVE state, saving only the FPSIMD subset of the register state. When we reload the state from memory we reenable SVE access traps, stopping tracking SVE until the task takes another SVE access trap. This means that for a task which is actively using SVE many blocking system calls will have the additional overhead of a SVE access trap. As SVE deployment is progressing we are seeing much wider use of the SVE instruction set, including performance optimised implementations of operations like memset() and memcpy(), which mean that even tasks which are not obviously floating point based can end up with substantial SVE usage. It does not, however, make sense to just unconditionally use the full SVE register state all the time since it is larger than the FPSIMD register state so there is overhead saving and restoring it on context switch and our requirement to flush the register state not shared with FPSIMD on syscall also creates a noticeable overhead on system call. I did some instrumentation which counted the number of SVE access traps and the number of times we loaded FPSIMD only register state for each task. Testing with Debian Bookworm this showed that during boot the overwhelming majority of tasks triggered another SVE access trap more than 50% of the time after loading FPSIMD only state with a substantial number near 100%, though some programs had a very small number of SVE accesses most likely from startup. There were few tasks in the range 5-45%, most tasks either used SVE frequently or used it only a tiny proportion of times. As expected older distributions which do not have the SVE performance work available showed no SVE usage in general applications. This indicates that there should be some useful benefit from reducing the number of SVE access traps for blocking system calls like we did for non blocking system calls in commit 8c845e273104 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch"). Let's do this with a timeout, when we take a SVE access trap record a jiffies after which we'll reeanble SVE traps then check this whenver we load a FPSIMD only floating point state from memory. If the time has passed then we reenable traps, otherwise we leave traps disabled and flush the non-shared register state like we would on trap. The timeout is currently set to a second, I pulled this number out of thin air so there is doubtless some room for tuning. This means that for a task which is actively using SVE the number of SVE access traps will be substantially reduced but applications which use SVE only very infrequently will avoid the overheads associated with tracking SVE state after a second. The extra cost from additional tracking of SVE state only occurs when a task is preempted so short running tasks should be minimally affected. There should be no functional change resulting from this, it is purely a performance optimisation. Signed-off-by: Mark Brown <broonie@kernel.org> --- Changes in v5: - Rebase onto v6.9-rc1. - Use a timeout rather than number of state loads to decide when to reenable traps. - Link to v4: https://lore.kernel.org/r/20240122-arm64-sve-trap-mitigation-v4-1-54e0d78a3ae9@kernel.org Changes in v4: - Rebase onto v6.8-rc1. - Link to v3: https://lore.kernel.org/r/20231113-arm64-sve-trap-mitigation-v3-1-4779c9382483@kernel.org Changes in v3: - Rebase onto v6.7-rc1. - Link to v2: https://lore.kernel.org/r/20230913-arm64-sve-trap-mitigation-v2-1-1bdeff382171@kernel.org Changes in v2: - Rebase onto v6.6-rc1. - Link to v1: https://lore.kernel.org/r/20230807-arm64-sve-trap-mitigation-v1-1-d92eed1d2855@kernel.org --- arch/arm64/include/asm/processor.h | 1 + arch/arm64/kernel/fpsimd.c | 42 ++++++++++++++++++++++++++++++++------ 2 files changed, 37 insertions(+), 6 deletions(-) --- base-commit: 4cece764965020c22cff7665b18a012006359095 change-id: 20230807-arm64-sve-trap-mitigation-2e7e2663c849 Best regards,