Messages in this thread | | | Date | Thu, 21 Nov 2013 16:10:05 -0800 | From | Jacob Pan <> | Subject | Re: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle implementations |
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On Thu, 21 Nov 2013 12:07:17 -0800 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 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 > currently powerclamp allow idle injection duration between 6 to 25ms. I guess that is short considering the stall check is in seconds? return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
BTW, by forcing intel_idle to use deepest c-states for idle injection thread the efficiency problem is gone. I am surprised that cpuidle would not pick the deepest c-states given powerclamp driver is asking for 6ms idle time and the wakeup latencies are in the usec. Anyway, for what i have tested so far powerclamp with this patchset can work as well as the code before.
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[Jacob Pan]
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