Messages in this thread | | | Date | Thu, 21 Nov 2013 12:07:17 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle implementations |
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On Thu, Nov 21, 2013 at 11:45:20AM -0800, Arjan van de Ven wrote: > On 11/21/2013 11:19 AM, Paul E. McKenney wrote: > >On Thu, Nov 21, 2013 at 08:21:03AM -0800, Arjan van de Ven wrote: > >>On 11/21/2013 8:07 AM, Paul E. McKenney wrote: > >>>As long as RCU has some reliable way to identify an idle task, I am > >>>good. But I have to ask -- why can't idle injection coordinate with > >>>the existing idle tasks rather than temporarily making alternative > >>>idle tasks? > >> > >>it's not a real idle. that's the whole problem of the situation. > >>to the rest of the OS, this is being BUSY (busy saving power using > >>a CPU instruction, but it might as well have been an mdelay() operation) > >>and it's also what end users expect; they want to be able to see > >>where there performance (read: cpu time in "top") is going. > > > >My concern is keeping RCU's books straight. Suppose that there is a need > >to call for idle in the middle of a preemptible RCU read-side critical > >section. Now, if that call for idle involves a context switch, all is > >well -- RCU will see the task as still being in its RCU read-side critical > >section, which means that it is OK for RCU to see the CPU as idle. > > > >However, if there is no context switch and RCU sees the CPU as idle, > >preemptible RCU could prematurely end the grace period. If there is no > >context switch and RCU sees the CPU as non-idle for too long, we start > >getting RCU CPU stall warning splats. > > > >Another approach would be to only inject idle when the CPU is not > >doing anything that could possibly be in an RCU read-side critical > >section. But things might get a bit hot in case of an overly > >long RCU read-side critical section. > > > >One approach that might work would be to hook into RCU's context-switch > >code going in and coming out, then telling RCU that the CPU is idle, > >even though top and friends see it as non-idle. This last is in fact > >similar to how RCU handles userspace execution for NO_HZ_FULL. > > > > so powerclamp and such are not "idle". > They are "busy" from everything except the lowest level of the CPU hardware. > once you start thinking of them as idle, all hell breaks lose in terms of implications > (including sysadmin visibility etc).... (hence some of the explosions in this thread > as well). > > but it's not "idle". > > it's "put the cpu in a low power state for a specified amount of time". sure it uses the same > instruction to do so that the idle loop uses. > > (now to make it messy, the current driver does a bunch of things similar to the idle loop > which is a mess and fair to be complained about)
Then from an RCU viewpoint, they need to be short in duration. Otherwise you risk getting CPU stall-warning explosions from RCU. ;-)
Thanx, Paul
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