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

[Jason Low]

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@xxxxxx>
> > ---
> >  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"?

> 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?

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