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

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Date:   Thu, 1 Sep 2022 10:48:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
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, 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.

-- Steve