linux-kernel - Re: [PATCH] memcg: remove trylock_page

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Message-Id: <20090421230147.eecfe82c.akpm@linux-foundation.org>
Date:	Tue, 21 Apr 2009 23:01:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	balbir@...ux.vnet.ibm.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: remove trylock_page_cgroup

On Wed, 22 Apr 2009 13:41:08 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> > I expect that it will reliably fail if the caller is running as
> > SCHED_FIFO and the machine is single-CPU, or if we're trying to yield
> > to a SCHED_OTHER task which is pinned to this CPU, etc.  The cond_resched()
> > won't work.
> > 
> Hm, signal_pending() is supported now (so special user scan use alaram())
> I used yield() before cond_resched() but I was told don't use it.
> Should I replace cond_resched() with congestion_wait(HZ/10) or some ?

msleep(1) would be typical.  That can also be used to give a
predictable number of seconds for the timeout.

If 1 millisecond is too coarse then it's possible to sleep for much
shorter intervals if the platform implements hi-res timers.  We don't
appear to have a handy interface to that (usleep, microsleep,
nanosleep, etc?).

And an attempt to sleep for 1us will fall back to 1/HZ if the platform
doesn't implement hi-res timers, so that loop will need to be turned
into a do {} while(!timer_after(jiffies, start))) thing.  Probably it
should be converted to that anyway, to be better behaved/predictable,
etc.

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