Re: [patch] sys_epoll_wait() timeout saga ...

From: Davide Libenzi (davidel_at_xmailserver.org)
Date: 09/24/05

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    Date:	Sat, 24 Sep 2005 08:10:32 -0700 (PDT)
To: Willy Tarreau <willy@w.ods.org>

    On Sat, 24 Sep 2005, Willy Tarreau wrote:

    > Yes it can, and that's why I said that gcc should send a warning when
    > comparing an int with something too large for an int. But I should have
    > forced the constant to be evaluated as long long. At the moment, the
    > constant cannot overflow, but it can reach a value so high that
    > timeout/1000 will never reach it. Example :
    > MAX_SCHEDULE_TIMEOUT=LONG_MAX
    > HZ=250
    > timeout=LONG_MAX-1
    > => timeout/1000 < MAX_SCHEDULE_TIMEOUT/HZ
    > but (timeout * HZ + 999) / 1000 will still overflow !
    >
    > So I finally think that the safest test would be to avoid the timeout
    > range which can overflow in the computation, using something like this
    > (but which will limit the timeout to 49 days on HZ=1000 machines) :
    >
    > + jtimeout = timeout < 0 || \
    > + timeout >= (1000ULL * MAX_SCHEDULE_TIMEOUT / HZ) || \
    > + timeout >= (LONG_MAX / HZ - 1000) ?
    > MAX_SCHEDULE_TIMEOUT: (timeout * HZ + 999) / 1000;
    >
    > as both are constants, they can be optimized. Otherwise, we can resort to
    > using a MAX() macro to reduce this to only one test which will catch all
    > corner cases.

    Using the MIN() macro would be better so we have a single check, and the
    compiler optimize that automatically. Or we can force 'timeout * HZ' to
    use ULL math. I don't think it makes a lot of difference for something
    that is in a likely sleep path ;)

    - Davide

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