linux-kernel - Re: [PATCH] perf/x86/intel: Avoid unnecessary reallocations of memory allocated in cpu hotplug prepare state

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Date:   Tue, 18 Dec 2018 20:45:32 +0800
From:   He Zhe <zhe.he@...driver.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
CC:     <acme@...nel.org>, <ak@...ux.intel.com>,
        <alexander.shishkin@...ux.intel.com>, <bp@...en8.de>,
        <hpa@...or.com>, <jolsa@...nel.org>, <jolsa@...hat.com>,
        <kan.liang@...ux.intel.com>, <mingo@...hat.com>,
        <namhyung@...nel.org>, <tglx@...utronix.de>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] perf/x86/intel: Avoid unnecessary reallocations of memory
 allocated in cpu hotplug prepare state



On 2018/12/18 19:47, Sebastian Andrzej Siewior wrote:
> On 2018-12-18 12:37:00 [+0100], Peter Zijlstra wrote:
>> On Tue, Dec 18, 2018 at 12:31:19PM +0100, Peter Zijlstra wrote:
>>> On Tue, Dec 18, 2018 at 12:16:37PM +0100, Sebastian Andrzej Siewior wrote:
>>>> On 2018-12-18 12:02:09 [+0100], Peter Zijlstra wrote:
>>>>> On Tue, Dec 18, 2018 at 06:30:33PM +0800, zhe.he@...driver.com wrote:
>>>>>> Besides, in preempt-rt full mode, the freeing can happen in atomic context and
>>>>>> thus cause the following BUG.
>>>>> Hurm, I though we fixed all those long ago..
>>>>>
>>>>> And no, the patch is horrible; that's what we have things like
>>>>> x86_pmu::cpu_dead() for.
>>>> ehm, you say we keep memory allocation +free on CPU up/down?
>>> Sure, why not?
> It does not seem to be useful to allocate & free memory which you need
> anyway. So you could avoid the refcnt for instance.
> Also I doubt the memory will remain unallocated for a longer period of
> time (like you would remove the CPU for good and not boot it again a
> minute later).
> *Maybe* it is different in cloud environment where you attach vcpus
> depending on guest load at runtime but still…
>
>> I suspect the below is all we really need.
> Zhe, could you please have a look?

This works in my environment, taking CPUs offline and online many times.

Zhe

>
> Sebastian
>