linux-kernel - Re: posix-cpu-timers revamp

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Message-Id: <1206132000.14638.42.camel@bobble.smo.corp.google.com>
Date:	Fri, 21 Mar 2008 13:40:00 -0700
From:	Frank Mayhar <fmayhar@...gle.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: posix-cpu-timers revamp

On Fri, 2008-03-21 at 00:18 -0700, Roland McGrath wrote:
> I think I misled you about the use of the it_*_expires fields, sorry.
> The task_struct.it_*_expires fields are used solely as a cache of the
> head of cpu_timers[].  Despite the poor choice of the same name, the
> signal_struct.it_*_expires fields serve a different purpose.  For an
> analogous cache of the soonest timer to expire, you need to add new
> fields.  The signal_struct.it_{prof,virt}_{expires,incr} fields hold
> the setitimer settings for ITIMER_{PROF,VTALRM}.  You can't change
> those in arm_timer.  For a quick cache you need a new field that is
> the sooner of it_foo_expires or the head cpu_timers[foo] expiry time.

Actually, after looking at the code again and thinking about it a bit,
it appears that the signal_struct.it_*_incr field holds the actual
interval as set by setitimer.  Initially the it_*_expires field holds
the expiration time as set by setitimer, but after the timer fires the
first time that value becomes <firing time>+it_*_incr.  In other words,
the first time it fires at the value set by setitimer() but from then on
it fires at a time indicated by whatever the time was the last time the
timer fired plus the value in it_*_incr.  This time is stored in
signal_struct.it_*_expires.

I guess I could be wrong about this, but it appears to be what the code
is doing.  If my analysis is correct, I really don't need a new field,
since the old fields work just fine.
-- 
Frank Mayhar <fmayhar@...gle.com>
Google, Inc.

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