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    SubjectRe: [PATCH RFC V1 0/5] Rationalize time keeping
    On Fri, Apr 27, 2012 at 03:49:51PM -0700, John Stultz wrote:
    > On 04/27/2012 01:12 AM, Richard Cochran wrote:
    > >* Performance Impacts
    > >** con
    > > - Small extra cost when reading the time (one integer addition plus
    > > one integer test).
    > This may not be so small when it comes to folks who are very
    > concerned about the clock_gettime hotpath.
    > Further, the correction will be needed to be made in the vsyscall
    > paths, which isn't done with your current patchset (causing userland
    > to see different time values then what kernel space calculates).

    John, now that you clarified the vDSO thing, I am very confused about
    this statement of yours. It appears that the vDSO data are updated
    when timekeeping_update() in timekeeper.c calls update_vsyscall().

    I think the hunk from patch #5, below, does in fact adjust the time
    value correctly before it gets handed off to the arch-specific
    update_vsyscall() to be copied into the vDSO page. So I'll make the
    claim that:

    1. We don't have to touch the vsyscall paths for this.
    2. This change does not affect vDSO performance at all.

    Would you mind taking another look at the patch?


    diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
    index 7941258..6cedf46 100644
    --- a/kernel/time/timekeeping.c
    +++ b/kernel/time/timekeeping.c
    @@ -212,11 +212,14 @@ static inline s64 timekeeping_get_ns_raw(void)
    /* must hold write on timekeeper.lock */
    static void timekeeping_update(bool clearntp)
    + struct timespec ts;
    if (clearntp) {
    timekeeper.ntp_error = 0;
    - update_vsyscall(&timekeeper.xtime, &timekeeper.wall_to_monotonic,
    + ts.tv_sec = timekeeper_utc_sec();
    + ts.tv_nsec = timekeeper.xtime.tv_nsec;
    + update_vsyscall(&ts, &timekeeper.wall_to_monotonic,
    timekeeper.clock, timekeeper.mult);

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