Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 1 Sep 2022 11:43:21 -0400
From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Suren Baghdasaryan <surenb@google.com>,
        akpm@linux-foundation.org, mhocko@suse.com, vbabka@suse.cz,
        hannes@cmpxchg.org, roman.gushchin@linux.dev, mgorman@suse.de,
        dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com,
        void@manifault.com, juri.lelli@redhat.com, ldufour@linux.ibm.com,
        peterx@redhat.com, david@redhat.com, axboe@kernel.dk,
        mcgrof@kernel.org, masahiroy@kernel.org, nathan@kernel.org,
        changbin.du@intel.com, ytcoode@gmail.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        bsegall@google.com, bristot@redhat.com, vschneid@redhat.com,
        cl@linux.com, penberg@kernel.org, iamjoonsoo.kim@lge.com,
        42.hyeyoo@gmail.com, glider@google.com, elver@google.com,
        dvyukov@google.com, shakeelb@google.com, songmuchun@bytedance.com,
        arnd@arndb.de, jbaron@akamai.com, rientjes@google.com,
        minchan@google.com, kaleshsingh@google.com,
        kernel-team@android.com, linux-mm@kvack.org, iommu@lists.linux.dev,
        kasan-dev@googlegroups.com, io-uring@vger.kernel.org,
        linux-arch@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-bcache@vger.kernel.org, linux-modules@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 03/30] Lazy percpu counters
Message-ID: <20220901154321.apyq7246srkjthfr@moria.home.lan>
References: <20220830214919.53220-1-surenb@google.com>
 <20220830214919.53220-4-surenb@google.com>
 <YxBWczNCbZbj+reQ@hirez.programming.kicks-ass.net>
 <20220901143219.n7jg7cbp47agqnwn@moria.home.lan>
 <20220901104839.5691e1c9@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220901104839.5691e1c9@gandalf.local.home>
Precedence: bulk

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@linux.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.