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Date: Wed, 24 Jul 2013 09:54:57 +0200
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Subject: Re: [Qemu-devel] [RFC 0/8] arm AioContext with its own timer stuff
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To: liu ping fan <qemulist@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Alex Bligh <alex@alex.org.uk>, Jan Kiszka <jan.kiszka@siemens.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>

Il 24/07/2013 09:43, liu ping fan ha scritto:
> Paid some time to dig the kernel code, and find out that the
> resolution lost by timeout of poll/select..etc is cause by the timeout
> is a slack region.
> See code in
> do_poll()
>    if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack))
> 
> Notice the slack param, it causes the lose of resolution.
> The process default slack time  inherits from init_task and it is
>       .timer_slack_ns = 50000, /* 50 usec default slack */
> But we can fix it by PR_SET_TIMERSLACK to decrease it for select/poll/...

Right, good catch.  I just learnt about PR_SET_TIMERSLACK. :)

Alex, can you add it to your series?  (Note that you must set a timer
slack of 1, because 0 is interpreted as "default").

> So ppoll with timerslack adjustment will meet our requirement. But
> what about other non-linux system?

They might have their own mechanism similar to PR_SET_TIMERSLACK.

Paolo