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Date: Tue, 18 May 2021 12:02:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Like Xu <like.xu@...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,
Kan Liang <kan.liang@...ux.intel.com>,
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 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?
> + }
> +}
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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