On Thu, 2015-08-27 at 14:53 +0200, Frederic Weisbecker wrote:
> On Wed, Aug 26, 2015 at 04:32:34PM -0700, Jason Low wrote:
> > On Thu, 2015-08-27 at 00:56 +0200, Frederic Weisbecker wrote:
> > > On Tue, Aug 25, 2015 at 08:17:48PM -0700, Jason Low wrote:
> > > > It was found while running a database workload on large systems that
> > > > significant time was spent trying to acquire the sighand lock.
> > > >
> > > > The issue was that whenever an itimer expired, many threads ended up
> > > > simultaneously trying to send the signal. Most of the time, nothing
> > > > happened after acquiring the sighand lock because another thread
> > > > had already sent the signal and updated the "next expire" time. The
> > > > fastpath_timer_check() didn't help much since the "next expire" time
> > > > was updated later.
> > > >
> > > > This patch addresses this by having the thread_group_cputimer structure
> > > > maintain a boolean to signify when a thread in the group is already
> > > > checking for process wide timers, and adds extra logic in the fastpath
> > > > to check the boolean.
> > > >
> > > > Signed-off-by: Jason Low <jason.l...@hp.com>
> > > > ---
> > > > include/linux/init_task.h | 1 +
> > > > include/linux/sched.h | 3 +++
> > > > kernel/time/posix-cpu-timers.c | 19 +++++++++++++++++--
> > > > 3 files changed, 21 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> > > > index d0b380e..3350c77 100644
> > > > --- a/include/linux/init_task.h
> > > > +++ b/include/linux/init_task.h
> > > > @@ -53,6 +53,7 @@ extern struct fs_struct init_fs;
> > > > .cputimer = {
> > > > \
> > > > .cputime_atomic = INIT_CPUTIME_ATOMIC,
> > > > \
> > > > .running = 0,
> > > > \
> > > > + .checking_timer = 0,
> > > > \
> > > > },
> > > > \
> > > > INIT_PREV_CPUTIME(sig)
> > > > \
> > > > .cred_guard_mutex =
> > > > \
> > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > index 119823d..a6c8334 100644
> > > > --- a/include/linux/sched.h
> > > > +++ b/include/linux/sched.h
> > > > @@ -619,6 +619,8 @@ struct task_cputime_atomic {
> > > > * @cputime_atomic: atomic thread group interval timers.
> > > > * @running: non-zero when there are timers running and
> > > > * @cputime receives updates.
> > > > + * @checking_timer: non-zero when a thread is in the process of
> > > > + * checking for thread group timers.
> > > > *
> > > > * This structure contains the version of task_cputime, above, that is
> > > > * used for thread group CPU timer calculations.
> > > > @@ -626,6 +628,7 @@ struct task_cputime_atomic {
> > > > struct thread_group_cputimer {
> > > > struct task_cputime_atomic cputime_atomic;
> > > > int running;
> > > > + int checking_timer;
> > >
> > > How about a flag in the "running" field instead?
> > >
> > > 1) Space in signal_struct is not as important as in task_strut but it
> > > still matters.
> >
> > George Spelvin suggested that we convert them to booleans which would
> > make them take up 2 bytes.
> >
> > > 2) We already read the "running" field locklessly. Adding a new field like
> > > checking_timer gets even more complicated. Ideally there should be at
> > > least a paired memory barrier between both. Let's just simplify that
> > > with a single field.
> >
> > hmmm, so having 1 "flag" where we access bits for the "running" and
> > "checking_timer"?
>
> Sure, like:
>
> #define CPUTIMER_RUNNING 0x1
> #define CPUTIMER_CHECKING 0x2
>
> struct thread_group_cputimer {
> struct task_cputime_atomic cputime_atomic;
> int status;
> }
>
> So from cputimer_running() you just need to check:
>
> if (cputimer->status & CPUTIMER_RUNNING)
>
> And from run_posix_cpu_timer() fast-path:
>
> if (cputimer->status == CPUTIMER_RUNNING)
>
> so that ignores CPUTIMER_CHECKING case.
Right, having just 1 "status" field can simply things a bit. The
(cputimer->status == CPUTIMER_RUNNING) check does appear misleading
though, since we're technically not only checking for if the "cputimer
is running".
Maybe something like:
int status = cputimer->status;
if ((status & CPUTIMER_RUNNING) && !(status & CPUTIMER_CHECKING))
makes it more obvious what's going on here.
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