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 13:34:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Tue, Dec 18, 2018 at 12:47:08PM +0100, 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.

It makes the code simpler and hotplug is not what you'd call a fast path
anyway.

> 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).

SMT disable comes to mind...

> *Maybe* it is different in cloud environment where you attach vcpus
> depending on guest load at runtime but still…

And that..