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Date: Tue, 18 May 2021 20:34:33 +0800
From: Like Xu <like.xu@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>,
Kan Liang <kan.liang@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...hat.com>,
Namhyung Kim <namhyung@...nel.org>,
linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
seanjc@...gle.com, x86@...nel.org
Subject: Re: [PATCH v2 2/2] perf/x86/lbr: Move cpuc->lbr_xsave allocation out
of sleeping region
On 2021/5/18 18:02, Peter Zijlstra wrote:
> On Fri, Apr 30, 2021 at 01:22:47PM +0800, Like Xu wrote:
>> If the kernel is compiled with the CONFIG_LOCKDEP option, the conditional
>> might_sleep_if() deep in kmem_cache_alloc() will generate the following
>> trace, and potentially cause a deadlock when another LBR event is added:
>
> ...
>
>> One of the solution is to use GFP_ATOMIC, but it will make the code less
>> reliable under memory pressue. Let's move the memory allocation out of
>> the sleeping region and put it into the intel_pmu_hw_config(). The LBR
>> xsave buffer is a per-CPU buffer, not a per-event buffer. This buffer is
>> allocated once when initializing the LBR event.
>>
>> The disadvantage of this fix is that the cpuc->lbr_xsave memory
>> will be allocated for each cpu like the legacy ds_buffer.
>>
>> Fixes: c085fb8774 ("perf/x86/intel/lbr: Support XSAVES for arch LBR read")
>> Tested-by: Kan Liang <kan.liang@...ux.intel.com>
>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>> ---
>> v1->v2 Changelog:
>> - Frob reserve_lbr_buffers() in intel_pmu_hw_config().
>>
>> arch/x86/events/intel/core.c | 2 ++
>> arch/x86/events/intel/lbr.c | 25 +++++++++++++++++++------
>> arch/x86/events/perf_event.h | 6 ++++++
>> 3 files changed, 27 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>> index 456aa6ffd9a1..19027aa01f82 100644
>> --- a/arch/x86/events/intel/core.c
>> +++ b/arch/x86/events/intel/core.c
>> @@ -3745,6 +3745,8 @@ static int intel_pmu_hw_config(struct perf_event *event)
>>
>> event->destroy = hw_perf_lbr_event_destroy;
>> }
>> +
>> + reserve_lbr_buffers(event);
>
> Ok, so this would actually work..
>
>> }
>>
>> if (event->attr.aux_output) {
>
>> @@ -722,6 +716,25 @@ void release_lbr_buffers(void)
>> }
>> }
>>
>> +void reserve_lbr_buffers(struct perf_event *event)
>> +{
>> + struct kmem_cache *kmem_cache;
>> + struct cpu_hw_events *cpuc;
>> + int cpu;
>> +
>> + if (!static_cpu_has(X86_FEATURE_ARCH_LBR))
>> + return;
>> +
>> + for_each_possible_cpu(cpu) {
>> + cpuc = per_cpu_ptr(&cpu_hw_events, cpu);
>> + kmem_cache = x86_get_pmu(cpu)->task_ctx_cache;
>> + if (kmem_cache && !cpuc->lbr_xsave && !event->attr.precise_ip)
>> + cpuc->lbr_xsave =
>> + kmem_cache_alloc_node(kmem_cache, GFP_KERNEL,
>> + cpu_to_node(cpu));
>
> (coding style fail)
>
> But then I looked at this function, and srlsy that !precise_ip is the
> only thing you need @event for? Why do we care?
Kan once commented, we only need to alloc the buffer for the non-PEBS
event. It seems the check "(cpuc->lbr_users != cpuc->lbr_pebs_users)"
is implicitly removed.
I think we still need to check !precision_ip, right ?
>
>> + }
>> +}
>
>
> Without that silly @event dependency you can go back to the original
> form, which makes much more sense since now reserve and release are
> symmetric.
>
> ---
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -396,10 +396,12 @@ int x86_reserve_hardware(void)
> if (!atomic_inc_not_zero(&pmc_refcount)) {
> mutex_lock(&pmc_reserve_mutex);
> if (atomic_read(&pmc_refcount) == 0) {
> - if (!reserve_pmc_hardware())
> + if (!reserve_pmc_hardware()) {
> err = -EBUSY;
> - else
> + } else {
> reserve_ds_buffers();
> + reserve_lbr_buffers();
> + }
> }
> if (!err)
> atomic_inc(&pmc_refcount);
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -658,7 +658,6 @@ static inline bool branch_user_callstack
>
> void intel_pmu_lbr_add(struct perf_event *event)
> {
> - struct kmem_cache *kmem_cache = event->pmu->task_ctx_cache;
> struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>
> if (!x86_pmu.lbr_nr)
> @@ -696,11 +695,6 @@ void intel_pmu_lbr_add(struct perf_event
> perf_sched_cb_inc(event->ctx->pmu);
> if (!cpuc->lbr_users++ && !event->total_time_running)
> intel_pmu_lbr_reset();
> -
> - if (static_cpu_has(X86_FEATURE_ARCH_LBR) &&
> - kmem_cache && !cpuc->lbr_xsave &&
> - (cpuc->lbr_users != cpuc->lbr_pebs_users))
> - cpuc->lbr_xsave = kmem_cache_alloc(kmem_cache, GFP_KERNEL);
> }
>
> void release_lbr_buffers(void)
> @@ -722,6 +716,26 @@ void release_lbr_buffers(void)
> }
> }
>
> +void reserve_lbr_buffers(void)
> +{
> + struct kmem_cache *kmem_cache;
> + struct cpu_hw_events *cpuc;
> + int cpu;
> +
> + if (!static_cpu_has(X86_FEATURE_ARCH_LBR))
> + return;
> +
> + for_each_possible_cpu(cpu) {
> + cpuc = per_cpu_ptr(&cpu_hw_events, cpu);
> + kmem_cache = x86_get_pmu(cpu)->task_ctx_cache;
> + if (!kmem_cache || cpuc->lbr_xsave)
> + continue;
> +
> + cpuc->lbr_xsave = kmem_cache_alloc_node(kmem_cache, GFP_KERNEL,
> + cpu_to_node(cpu));
> + }
> +}
> +
> void intel_pmu_lbr_del(struct perf_event *event)
> {
> struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -1244,6 +1244,8 @@ void reserve_ds_buffers(void);
>
> void release_lbr_buffers(void);
>
> +void reserve_lbr_buffers(void);
> +
> extern struct event_constraint bts_constraint;
> extern struct event_constraint vlbr_constraint;
>
> @@ -1393,6 +1395,10 @@ static inline void release_lbr_buffers(v
> {
> }
>
> +static inline void reserve_lbr_buffers(void)
> +{
> +}
> +
> static inline int intel_pmu_init(void)
> {
> return 0;
>
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