Messages in this thread | | | Date | Fri, 27 Sep 2013 11:59:03 +0100 | From | Will Deacon <> | Subject | Re: [RFT PATCH v1 6/7] amd64: avoid saving and restoring FPSIMD registers until threads access them |
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On Fri, Sep 27, 2013 at 09:04:46AM +0100, Jiang Liu wrote: > From: Jiang Liu <jiang.liu@huawei.com> > > 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...
> Signed-off-by: Jiang Liu <jiang.liu@huawei.com> > Cc: Jiang Liu <liuj97@gmail.com> > --- > 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
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