Messages in this thread | | | Subject | Re: [PATCH 08/10] posix-cpu-timers: Migrate to use new tick dependency mask model | From | Chris Metcalf <> | Date | Thu, 30 Jul 2015 15:52:54 -0400 |
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On 07/30/2015 03:45 PM, Frederic Weisbecker wrote: > >>>> You mentioned needing two fields, for task and for process, but in >>>> fact let's just add the one field to the one thing that needs it and >>>> not worry about additional possible future needs. And note that it's >>>> the task_struct->signal where we need to add the field for posix cpu >>>> timers (the signal_struct) since that's where the sharing occurs, and >>>> given CLONE_SIGHAND I imagine it could be different from the general >>>> "process" model anyway. >>> Well, posix cpu timers can be install per process (signal struct) or >>> per thread (task struct). >>> >>> But we can certainly simplify that with a per process flag and expand >>> the thread dependency to the process scope. >>> >>> Still there is the issue of telling the CPUs where a process runs when >>> a posix timer is installed there. There is no process-like tsk->cpus_allowed. >>> Either we send an IPI everywhere like we do now or we iterate through all >>> threads in the process to OR all their cpumasks in order to send that IPI. >> Is there a reason the actual timer can't run on a housekeeping >> core? Then when it does wake_up_process() or whatever, the >> specific target task will get an IPI to wake up at that point. > It makes sense if people run posix cpu timers on nohz full CPUs. But nobody > reported such usecase yet.
The corner case I was trying to address with my comment above is when a process includes both housekeeping and nohz_full threads. This is generally a bad idea in my experience, but our customers do this sometimes (usually because they're porting a big pile of code from somewhere else), and if so it would be good if we didn't have to keep every thread in that task ticking; presumably it is enough to ensure the timer lands on a housekeeping core instead, possibly the one for the non-fast-path thread in question, and then the regular IPIs from wake_up_process() will be sufficient if for some lame reason the signal ends up handled on a nohz_full core.
-- Chris Metcalf, EZChip Semiconductor http://www.ezchip.com
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