Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] perf/x86/intel: Avoid PEBS_ENABLE MSR access in PMI
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     acme@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org,
        ak@linux.intel.com, eranian@google.com
References: <20191115133917.24424-1-kan.liang@linux.intel.com>
 <20191115140739.GM4131@hirez.programming.kicks-ass.net>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <c0f562aa-39f3-1291-edd7-17c46178d1e3@linux.intel.com>
Date:   Fri, 15 Nov 2019 09:46:25 -0500
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.1
MIME-Version: 1.0
In-Reply-To: <20191115140739.GM4131@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 11/15/2019 9:07 AM, Peter Zijlstra wrote:
> On Fri, Nov 15, 2019 at 05:39:17AM -0800, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The perf PMI handler, intel_pmu_handle_irq(), currently does
>> unnecessary MSR accesses when PEBS is enabled.
>>
>> When entering the handler, global ctrl is explicitly disabled. All
>> counters do not count anymore. It doesn't matter if the PEBS is
>> enabled or disabled. Furthermore, cpuc->pebs_enabled is not changed
>> in PMI. The PEBS status doesn't change. The PEBS_ENABLE MSR doesn't need
>> to be changed either.
> 
> PMI can throttle, and iirc x86_pmu_stop() ends up in
> intel_pmu_pebs_disable()
>

Right, the declaration is inaccurate. I will fix it in v2.
But the patch still works for the case of PMI throttle.
The intel_pmu_pebs_disable() will update cpuc->pebs_enabled and 
unconditionally modify MSR_IA32_PEBS_ENABLE.
When exiting the handler, current perf re-write the PEBS MSR according 
to the updated cpuc->pebs_enabled, which is still unnecessary.

Thanks,
Kan