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

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Message-ID: <20181218114708.p6fpz447brob4kzz@linutronix.de>
Date:   Tue, 18 Dec 2018 12:47:08 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     zhe.he@...driver.com, 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 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?

Sebastian