From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT
Date: Fri, 10 Dec 2021 17:34:33 +0100 [thread overview]
Message-ID: <YbOBmYrInQVI+VOR@linutronix.de> (raw)
In-Reply-To: <80ee87bb-f36c-4a16-9095-43ea84818375@redhat.com>
On 2021-12-10 11:29:31 [-0500], Waiman Long wrote:
>
> On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
> > On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
> > …
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > …
> > > @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > > struct memcg_stock_pcp *stock;
> > > unsigned long flags;
> > > - local_irq_save(flags);
> > > + local_lock_irqsave(&memcg_stock.lock, flags);
> > Why is this one using the lock? It isn't accessing irq_obj, right?
> Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
> structure which include irq_obj. Sometimes, data in irq_obj (or task_obj)
> will get transfer to nr_pages and vice versa. So it is easier to use one
> single lock for the whole thing.
This needs way better documentation what protects what any why.
I don't like the quick slapping for RT only usage without any kind of
explanation. Once you think you know it is irq_obj only you end up here
where you have the lock again for no obvious reason.
>
> Cheers,
> Longman
Sebastian
WARNING: multiple messages have this Message-ID (diff)
From: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
To: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>,
Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
Vladimir Davydov
<vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
Andrew Morton
<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>,
Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT
Date: Fri, 10 Dec 2021 17:34:33 +0100 [thread overview]
Message-ID: <YbOBmYrInQVI+VOR@linutronix.de> (raw)
In-Reply-To: <80ee87bb-f36c-4a16-9095-43ea84818375-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
On 2021-12-10 11:29:31 [-0500], Waiman Long wrote:
>
> On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
> > On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
> > …
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > …
> > > @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > > struct memcg_stock_pcp *stock;
> > > unsigned long flags;
> > > - local_irq_save(flags);
> > > + local_lock_irqsave(&memcg_stock.lock, flags);
> > Why is this one using the lock? It isn't accessing irq_obj, right?
> Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
> structure which include irq_obj. Sometimes, data in irq_obj (or task_obj)
> will get transfer to nr_pages and vice versa. So it is easier to use one
> single lock for the whole thing.
This needs way better documentation what protects what any why.
I don't like the quick slapping for RT only usage without any kind of
explanation. Once you think you know it is irq_obj only you end up here
where you have the lock again for no obvious reason.
>
> Cheers,
> Longman
Sebastian
next prev parent reply other threads:[~2021-12-10 16:34 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-10 2:52 [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT Waiman Long
2021-12-10 2:52 ` Waiman Long
2021-12-10 13:01 ` Sebastian Andrzej Siewior
2021-12-10 13:01 ` Sebastian Andrzej Siewior
2021-12-10 16:29 ` Waiman Long
2021-12-10 16:29 ` Waiman Long
2021-12-10 16:34 ` Sebastian Andrzej Siewior [this message]
2021-12-10 16:34 ` Sebastian Andrzej Siewior
2021-12-10 16:37 ` Waiman Long
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