Re: [PATCH v2 2/2] perf/x86/lbr: Move cpuc->lbr_xsave allocation out of sleeping region

[Like Xu]

Hi Peter, do you have any comments on this version ?

On 2021/4/30 13:22, 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:
>
> [  243.115549] BUG: sleeping function called from invalid context at include/linux/sched/mm.h:196
> [  243.117576] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 839, name: perf
> [  243.119326] INFO: lockdep is turned off.
> [  243.120249] irq event stamp: 0
> [  243.120967] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
> [  243.122415] hardirqs last disabled at (0): [<ffffffff810d9bf5>] copy_process+0xa45/0x1dc0
> [  243.124302] softirqs last  enabled at (0): [<ffffffff810d9bf5>] copy_process+0xa45/0x1dc0
> [  243.126255] softirqs last disabled at (0): [<0000000000000000>] 0x0
> [  243.128119] CPU: 0 PID: 839 Comm: perf Not tainted 5.11.0-rc4-guest+ #8
> [  243.129654] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
> [  243.131520] Call Trace:
> [  243.132112]  dump_stack+0x8d/0xb5
> [  243.132896]  ___might_sleep.cold.106+0xb3/0xc3
> [  243.133984]  slab_pre_alloc_hook.constprop.85+0x96/0xd0
> [  243.135208]  ? intel_pmu_lbr_add+0x152/0x170
> [  243.136207]  kmem_cache_alloc+0x36/0x250
> [  243.137126]  intel_pmu_lbr_add+0x152/0x170
> [  243.138088]  x86_pmu_add+0x83/0xd0
> [  243.138889]  ? lock_acquire+0x158/0x350
> [  243.139791]  ? lock_acquire+0x158/0x350
> [  243.140694]  ? lock_acquire+0x158/0x350
> [  243.141625]  ? lock_acquired+0x1e3/0x360
> [  243.142544]  ? lock_release+0x1bf/0x340
> [  243.143726]  ? trace_hardirqs_on+0x1a/0xd0
> [  243.144823]  ? lock_acquired+0x1e3/0x360
> [  243.145742]  ? lock_release+0x1bf/0x340
> [  243.147107]  ? __slab_free+0x49/0x540
> [  243.147966]  ? trace_hardirqs_on+0x1a/0xd0
> [  243.148924]  event_sched_in.isra.129+0xf8/0x2a0
> [  243.149989]  merge_sched_in+0x261/0x3e0
> [  243.150889]  ? trace_hardirqs_on+0x1a/0xd0
> [  243.151869]  visit_groups_merge.constprop.135+0x130/0x4a0
> [  243.153122]  ? sched_clock_cpu+0xc/0xb0
> [  243.154023]  ctx_sched_in+0x101/0x210
> [  243.154884]  ctx_resched+0x6f/0xc0
> [  243.155686]  perf_event_exec+0x21e/0x2e0
> [  243.156641]  begin_new_exec+0x5e5/0xbd0
> [  243.157540]  load_elf_binary+0x6af/0x1770
> [  243.158478]  ? __kernel_read+0x19d/0x2b0
> [  243.159977]  ? lock_acquire+0x158/0x350
> [  243.160876]  ? __kernel_read+0x19d/0x2b0
> [  243.161796]  bprm_execve+0x3c8/0x840
> [  243.162638]  do_execveat_common.isra.38+0x1a5/0x1c0
> [  243.163776]  __x64_sys_execve+0x32/0x40
> [  243.164676]  do_syscall_64+0x33/0x40
> [  243.165514]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [  243.166746] RIP: 0033:0x7f6180a26feb
> [  243.167590] Code: Unable to access opcode bytes at RIP 0x7f6180a26fc1.
> [  243.169097] RSP: 002b:00007ffc6558ce18 EFLAGS: 00000202 ORIG_RAX: 000000000000003b
> [  243.170844] RAX: ffffffffffffffda RBX: 00007ffc65592d30 RCX: 00007f6180a26feb
> [  243.172514] RDX: 000055657f408dc0 RSI: 00007ffc65592410 RDI: 00007ffc65592d30
> [  243.174162] RBP: 00007ffc6558ce80 R08: 00007ffc6558cde0 R09: 0000000000000000
> [  243.176042] R10: 0000000000000008 R11: 0000000000000202 R12: 00007ffc65592410
> [  243.177696] R13: 000055657f408dc0 R14: 0000000000000001 R15: 00007ffc65592410
>
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> ---
> 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);
>   	}
>   
>   	if (event->attr.aux_output) {
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index bb4486c4155a..1c39155c9f67 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -658,7 +658,6 @@ static inline bool branch_user_callstack(unsigned br_sel)
>   
>   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 *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 +716,25 @@ 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));
> +	}
> +}
> +
>   void intel_pmu_lbr_del(struct perf_event *event)
>   {
>   	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index 27fa85e7d4fd..727e5490e78c 100644
> --- 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(struct perf_event *event);
> +
>   extern struct event_constraint bts_constraint;
>   extern struct event_constraint vlbr_constraint;
>   
> @@ -1393,6 +1395,10 @@ static inline void release_lbr_buffers(void)
>   {
>   }
>   
> +static inline void reserve_lbr_buffers(struct perf_event *event)
> +{
> +}
> +
>   static inline int intel_pmu_init(void)
>   {
>   	return 0;