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Subject: Re: [PATCH v4 RESEND 3/5] perf/x86/lbr: Move cpuc->lbr_xsave
 allocation out of sleeping region
To:     Peter Zijlstra <peterz@infradead.org>,
        Like Xu <like.xu@linux.intel.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org
References: <20210322060635.821531-1-like.xu@linux.intel.com>
 <20210322060635.821531-4-like.xu@linux.intel.com>
 <20210323214140.GE4746@worktop.programming.kicks-ass.net>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <b9532af0-4539-960d-dd7a-78c8731ba2f6@linux.intel.com>
Date:   Tue, 23 Mar 2021 19:03:24 -0400
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On 3/23/2021 5:41 PM, Peter Zijlstra wrote:
> On Mon, Mar 22, 2021 at 02:06:33PM +0800, Like Xu wrote:
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 18df17129695..a4ce669cc78d 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -373,7 +373,7 @@ set_ext_hw_attr(struct hw_perf_event *hwc, struct perf_event *event)
>>   	return x86_pmu_extra_regs(val, event);
>>   }
>>   
>> -int x86_reserve_hardware(void)
>> +int x86_reserve_hardware(struct perf_event *event)
>>   {
>>   	int err = 0;
>>   
>> @@ -382,8 +382,10 @@ int x86_reserve_hardware(void)
>>   		if (atomic_read(&pmc_refcount) == 0) {
>>   			if (!reserve_pmc_hardware())
>>   				err = -EBUSY;
>> -			else
>> +			else {
>>   				reserve_ds_buffers();
>> +				reserve_lbr_buffers(event);
>> +			}
>>   		}
>>   		if (!err)
>>   			atomic_inc(&pmc_refcount);
> 
> This makes absolutely no sense what so ever. This is only executed for
> the first event that gets here.
> 

The LBR xsave buffer is a per-CPU buffer, not a per-event buffer. I 
think we only need to allocate the buffer once when initializing the 
first event.

Thanks,
Kan