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Date: Thu, 6 Feb 2014 16:09:32 +0100
From: Stephane Eranian <eranian@...gle.com>
To: "Yan, Zheng" <zheng.z.yan@...el.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...nel.org>,
Arnaldo Carvalho de Melo <acme@...radead.org>,
Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 09/14] perf, x86: Save/resotre LBR stack during context switch
On Wed, Feb 5, 2014 at 6:45 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
>> When the LBR call stack is enabled, it is necessary to save/restore
>> the LBR stack on context switch. The solution is saving/restoring
>> the LBR stack to/from task's perf event context.
>>
>> The LBR stack is saved/restored only when there are events that use
>> the LBR call stack. If no event uses LBR call stack, the LBR stack
>> is reset when task is scheduled in.
>>
>> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
>> ---
>> arch/x86/kernel/cpu/perf_event_intel_lbr.c | 80 ++++++++++++++++++++++++------
>> 1 file changed, 66 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
>> index 2137a9f..51e1842 100644
>> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
>> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
>> @@ -187,18 +187,82 @@ void intel_pmu_lbr_reset(void)
>> intel_pmu_lbr_reset_64();
>> }
>>
>> +/*
>> + * TOS = most recently recorded branch
>> + */
>> +static inline u64 intel_pmu_lbr_tos(void)
>> +{
>> + u64 tos;
>> + rdmsrl(x86_pmu.lbr_tos, tos);
>> + return tos;
>> +}
>> +
>> +enum {
>> + LBR_UNINIT,
>> + LBR_NONE,
>> + LBR_VALID,
>> +};
>> +
> I don't see where the x86_perf_task_context struct gets initialized with
> your task_ctx_data/task_ctx_size mechanism. You are relying on 0
> as a valid default value. But if later more fields are needed and they need
> non-zero init values, it will be easy to forget.....
>
> So I think you need to provide a callback from alloc_perf_context().
> Should have mentioned that in Patch 05/14.
>
>> +static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
>> +{
>> + int i;
>> + unsigned lbr_idx, mask = x86_pmu.lbr_nr - 1;
>> + u64 tos = intel_pmu_lbr_tos();
>> +
>> + for (i = 0; i < x86_pmu.lbr_nr; i++) {
>> + lbr_idx = (tos - i) & mask;
>> + wrmsrl(x86_pmu.lbr_from + lbr_idx, task_ctx->lbr_from[i]);
>> + wrmsrl(x86_pmu.lbr_to + lbr_idx, task_ctx->lbr_to[i]);
>> + }
>> + task_ctx->lbr_stack_state = LBR_NONE;
>> +}
>> +
>> +static void __intel_pmu_lbr_save(struct x86_perf_task_context *task_ctx)
>> +{
>> + int i;
>> + unsigned lbr_idx, mask = x86_pmu.lbr_nr - 1;
>> + u64 tos = intel_pmu_lbr_tos();
>> +
>> + for (i = 0; i < x86_pmu.lbr_nr; i++) {
>> + lbr_idx = (tos - i) & mask;
>> + rdmsrl(x86_pmu.lbr_from + lbr_idx, task_ctx->lbr_from[i]);
>> + rdmsrl(x86_pmu.lbr_to + lbr_idx, task_ctx->lbr_to[i]);
>> + }
>> + task_ctx->lbr_stack_state = LBR_VALID;
>> +}
>> +
>> +
>> void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>> {
>> + struct cpu_hw_events *cpuc;
>> + struct x86_perf_task_context *task_ctx;
>> +
>> if (!x86_pmu.lbr_nr)
>> return;
>>
>> + cpuc = &__get_cpu_var(cpu_hw_events);
>> + task_ctx = ctx ? ctx->task_ctx_data : NULL;
>> +
>> +
>> /*
>> * It is necessary to flush the stack on context switch. This happens
>> * when the branch stack does not tag its entries with the pid of the
>> * current task.
>> */
>> - if (sched_in)
>> - intel_pmu_lbr_reset();
>> + if (sched_in) {
>> + if (!task_ctx ||
>> + !task_ctx->lbr_callstack_users ||
>> + task_ctx->lbr_stack_state != LBR_VALID)
>> + intel_pmu_lbr_reset();
>> + else
>> + __intel_pmu_lbr_restore(task_ctx);
>> + } else if (task_ctx) {
>> + if (task_ctx->lbr_callstack_users &&
>> + task_ctx->lbr_stack_state != LBR_UNINIT)
>> + __intel_pmu_lbr_save(task_ctx);
>> + else
>> + task_ctx->lbr_stack_state = LBR_NONE;
>> + }
>> }
>>
> There ought to be a better way of structuring this if/else. It is
> ugly.
>
Second thought on this. I am not sure I understand why the
test has to be so complex including on the save() side.
if (sched_in) {
if (task_ctx && lbr_callstack_users)
restore()
else
reset
} else { /* sched_out */
if (task_ctx && lbr_callstack_users)
save()
}
If you have lbr_callstack_users, then you need to save/restore.
Looks like you are trying to prevent from double sched-in or
double sched-out. Can this happen?
In other words, I am not sure I understand the need for the
lbr_state here.
>> static inline bool branch_user_callstack(unsigned br_sel)
>> @@ -267,18 +331,6 @@ void intel_pmu_lbr_disable_all(void)
>> __intel_pmu_lbr_disable();
>> }
>>
>> -/*
>> - * TOS = most recently recorded branch
>> - */
>> -static inline u64 intel_pmu_lbr_tos(void)
>> -{
>> - u64 tos;
>> -
>> - rdmsrl(x86_pmu.lbr_tos, tos);
>> -
>> - return tos;
>> -}
>> -
>> static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
>> {
>> unsigned long mask = x86_pmu.lbr_nr - 1;
>> --
>> 1.8.4.2
>>
--
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