[RFT PATCH v1 6/7] amd64: avoid saving and restoring FPSIMD registers until threads access them
From: Will Deacon <hidden>
Date: 2013-09-27 10:59:38
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On Fri, Sep 27, 2013 at 09:04:46AM +0100, Jiang Liu wrote:
From: Jiang Liu <redacted> Use PF_USED_MATH flag to mark whether the thread has accessed any FPSIMD registers, so we could avoid saving and restroing FPSIMD registers until threads access them. This may improve performance when lazy FPSIMD restore is disabled.
Hehe, the subject made me smile :) I suppose that means I have to give a semi-useful review for the patch...
quoted hunk ↗ jump to hunk
Signed-off-by: Jiang Liu <redacted> Cc: Jiang Liu <redacted> --- arch/arm64/kernel/fpsimd.c | 38 +++++++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 15 deletions(-)diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 267e54a..a81af5f 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c@@ -99,7 +99,8 @@ void fpsimd_disable_lazy_restore(void) * If lazy mode is enabled, caller needs to disable preemption * when calling fpsimd_load_state_lazy() and fpsimd_save_state_lazy(). */ -static void fpsimd_load_state_lazy(struct fpsimd_state *state) +static void fpsimd_load_state_lazy(struct fpsimd_state *state, + struct task_struct *tsk) { /* Could we reuse the hardware context? */ if (state->last_cpu == smp_processor_id() &&@@ -109,13 +110,19 @@ static void fpsimd_load_state_lazy(struct fpsimd_state *state) if (static_key_false(&fpsimd_lazy_mode)) { fpsimd_clear_on_hw(state); fpsimd_enable_trap(); - } else { + } else if (tsk_used_math(tsk)) { + fpsimd_disable_trap(); fpsimd_load_state(state); + } else { + fpsimd_enable_trap();
One thing worth checking in sequences like this is that you have the relevant memory barriers (isb instructions) to ensure that the CPU is synchronised wrt side-effects from the msr instructions. *Some* operations are self-synchronising, but I don't think this is the case for fpsimd in v8 (although I haven't re-checked). Your earlier patch (3/7) doesn't seem to have any of these barriers. Will