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From: Michal Hocko <mhocko@kernel.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Chris Metcalf <chris.d.metcalf@gmail.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	linux-mm@kvack.org, Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH] mm/swap.c: workaround for_each_cpu() bug on UP kernel.
Date: Tue, 12 Feb 2019 12:21:17 +0100	[thread overview]
Message-ID: <20190212112117.GT15609@dhcp22.suse.cz> (raw)
In-Reply-To: <82168e14-8a89-e6ac-1756-e473e9c21616@i-love.sakura.ne.jp>

On Tue 12-02-19 19:25:46, Tetsuo Handa wrote:
> On 2019/02/12 19:11, Michal Hocko wrote:
> > This patch is ugly as hell! I do agree that for_each_cpu not working on
> > CONFIG_SMP=n sucks but why do we even care about lru_add_drain_all when
> > there is a single cpu? Why don't we simply do
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index aa483719922e..952f24b09070 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -660,6 +660,7 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
> >  
> >  static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
> >  
> > +#ifdef CONFIG_SMP
> >  /*
> >   * Doesn't need any cpu hotplug locking because we do rely on per-cpu
> >   * kworkers being shut down before our page_alloc_cpu_dead callback is
> > @@ -702,6 +703,10 @@ void lru_add_drain_all(void)
> >  
> >  	mutex_unlock(&lock);
> >  }
> > +#else
> > +#define lru_add_drain_all() lru_add_drain()
> > +
> > +#endif
> 
> If there is no need to evaluate the "if" conditions, I'm fine with this shortcut.

lru_add_drain does drain only pagevecs which have pages and so we do not
really have to duplicate the check. There is also no need to defer the
execution to the workqueue for a local cpu. So we are left with only the
lock to prevent parallel execution but the preemption disabling acts the
same purpose on UP so the approach should be equivalent from the
correctness point of view.
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2019-02-12 11:21 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-07  9:53 Tetsuo Handa
2019-02-07 14:07 ` Guenter Roeck
2019-02-07 14:18 ` William Kucharski
2019-02-12 10:11 ` Michal Hocko
2019-02-12 10:25   ` Tetsuo Handa
2019-02-12 11:21     ` Michal Hocko [this message]
2019-02-12 11:29       ` Michal Hocko
2019-02-12 11:37         ` Tetsuo Handa
2019-02-12 21:06         ` Andrew Morton
2019-02-13 12:43           ` Michal Hocko
2019-02-13 21:37             ` Andrew Morton
2019-02-12 23:19         ` [PATCH] mm: handle lru_add_drain_all for UP properly kbuild test robot
2019-02-12 23:48         ` kbuild test robot

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