Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention

Frederic Weisbecker
2015-08-27T12:53:08Z

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.low2@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.

>
> > Now concerning the solution for your problem, I'm a bit uncomfortable with
> > lockless magics like this. When the thread sets checking_timer to 1, there is
> > no guarantee that the other threads in the process will see it "fast" enough
> > to avoid the slow path checks. Then there is also the risk that the threads
> > don't see "fast" enough that checking_timers has toggled to 0 and as a result
> > a timer may expire late. Now the lockless access of "running" already induces
> > such race. So if it really solves issues in practice, why not.
>
> Perhaps to be safer, we use something like load_acquire() and
> store_release() for accessing both the ->running and ->checking_timer
> fields?

Well it depends against what we want to order them. If it's a single field
we don't need to order them together at least.