Messages in this thread | | | Date | Mon, 7 Nov 2022 11:11:15 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v4 05/16] add_timer_on(): Make sure callers have TIMER_PINNED flag |
| |
On Mon, Nov 07, 2022 at 09:11:11AM +0100, Anna-Maria Behnsen wrote: > On Fri, 4 Nov 2022, Frederic Weisbecker wrote: > > > On Fri, Nov 04, 2022 at 03:57:26PM +0100, Anna-Maria Behnsen wrote: > > > The implementation of the hierachical timer pull model will change the > > > timer bases per CPU. Timers, that have to expire on a specific CPU, require > > > the TIMER_PINNED flag. Otherwise they will be queued on the dedicated CPU > > > but in global timer base and those timers could also expire on other > > > CPUs. Timers with TIMER_DEFERRABLE flag end up in a separate base anyway > > > and are executed on the local CPU only. > > > > > > Therefore add the missing TIMER_PINNED flag for those callers who use > > > add_timer_on() without the flag. No functional change. > > > > You're fixing the current callers but what about the future ones? > > > > add_timer_on() should always guarantee that a timer runs on the > > right destination, which is not the case after your patchset if the > > timer hasn't been set to TIMER_PINNED. > > > > Therefore I think we should either have: > > > > * add_timer_on() enforce TIMER_PINNED (doesn't work because if the timer is > > later called with mod_timer(), we should expect it to run anywhere) > > > > or > > > > * add_timer_on() warns if !TIMER_PINNED > > This is already part of the last patch of the queue where also the > crystalball logic is removed. But the patch where I added the WARN_ONCE() > might be the wrong patch, it should be better part of the next patch where > the new timer bases are introduced.
Ok.
> > > or > > > > * have an internal flag TIMER_LOCAL, that is turned on when > > add_timer_on() is called or add_timer()/mod_timer() is called > > on a TIMER_PINNED. Otherwise it is turned off. > > > > The last solution should work with existing API and you don't need to > > chase the current and future users of add_timer_on(). > > With the last approach it doesn't matter how the timer is setup. Everything > is done by timer code implicitly. When a future caller uses add_timer_on() > and wants to modfiy this "implicitly pinned timer", he will call > mod_timer() and the timer is no longer pinned (if it do not end up in the > same bucket it was before). For a user this does not seems to be very > obvious, or am I wrong?
That's right indeed.
> > But if the caller sets up the timer correctly we do not need this extra > timer flag. With the WARN_ONCE() in place, callers need to do the timer > setup properly and it is more clear to the caller what should be done.
Yeah that sounds better.
> BTW, the hunk in this patch for the workqueue is also not a final fix in my > opinion. I'm preparing a cleanup queue (it's part of the deferrable cleanup > queue), where I want to set the timer flags properly when > initializing/defining the workers. I should have added a comment here...
Ok, if we have some pinned initializers such as DECLARE_DELAYED_WORK_PINNED() and DECLARE_DEFERRABKE_WORK_PINNED() then I think that cleans the situation.
Sounds good, thanks!
> > Thanks, > > Anna-Maria >
| |