linux-kernel - Re: [RFC PATCH 03/30] Lazy percpu counters

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Message-ID: <20220901154321.apyq7246srkjthfr@moria.home.lan>
Date:   Thu, 1 Sep 2022 11:43:21 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
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Subject: Re: [RFC PATCH 03/30] Lazy percpu counters

On Thu, Sep 01, 2022 at 10:48:39AM -0400, Steven Rostedt wrote:
> On Thu, 1 Sep 2022 10:32:19 -0400
> Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> 
> > On Thu, Sep 01, 2022 at 08:51:31AM +0200, Peter Zijlstra wrote:
> > > On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:  
> > > > +static void lazy_percpu_counter_switch_to_pcpu(struct raw_lazy_percpu_counter *c)
> > > > +{
> > > > +	u64 __percpu *pcpu_v = alloc_percpu_gfp(u64, GFP_ATOMIC|__GFP_NOWARN);  
> > > 
> > > Realize that this is incorrect when used under a raw_spinlock_t.  
> > 
> > Can you elaborate?
> 
> All allocations (including GFP_ATOMIC) grab normal spin_locks. When
> PREEMPT_RT is configured, normal spin_locks turn into a mutex, where as
> raw_spinlock's do not.
> 
> Thus, if this is done within a raw_spinlock with PREEMPT_RT configured, it
> can cause a schedule while holding a spinlock.

Thanks, I think we should be good here but I'll document it anyways.