Messages in this thread | | | Date | Fri, 3 Mar 2023 15:44:13 -0800 | From | Jakub Kicinski <> | Subject | Re: [PATCH 2/3] softirq: avoid spurious stalls due to need_resched() |
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On Fri, 3 Mar 2023 15:36:27 -0800 Paul E. McKenney wrote: > On Fri, Mar 03, 2023 at 02:37:39PM -0800, Paul E. McKenney wrote: > > On Fri, Mar 03, 2023 at 01:31:43PM -0800, Jakub Kicinski wrote: > > > Now - now about the max loop count. I ORed the pending softirqs every > > > time we get to the end of the loop. Looks like vast majority of the > > > loop counter wake ups are exclusively due to RCU: > > > > > > @looped[512]: 5516 > > > > > > Where 512 is the ORed pending mask over all iterations > > > 512 == 1 << RCU_SOFTIRQ. > > > > > > And they usually take less than 100us to consume the 10 iterations. > > > Histogram of usecs consumed when we run out of loop iterations: > > > > > > [16, 32) 3 | | > > > [32, 64) 4786 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > > > [64, 128) 871 |@@@@@@@@@ | > > > [128, 256) 34 | | > > > [256, 512) 9 | | > > > [512, 1K) 262 |@@ | > > > [1K, 2K) 35 | | > > > [2K, 4K) 1 | | > > > > > > Paul, is this expected? Is RCU not trying too hard to be nice? > > > > This is from way back in the day, so it is quite possible that better > > tuning and/or better heuristics should be applied. > > > > On the other hand, 100 microseconds is a good long time from an > > CONFIG_PREEMPT_RT=y perspective! > > > > > # cat /sys/module/rcutree/parameters/blimit > > > 10 > > > > > > Or should we perhaps just raise the loop limit? Breaking after less > > > than 100usec seems excessive :( > > > > But note that RCU also has rcutree.rcu_divisor, which defaults to 7. > > And an rcutree.rcu_resched_ns, which defaults to three milliseconds > > (3,000,000 nanoseconds). This means that RCU will do: > > > > o All the callbacks if there are less than ten. > > > > o Ten callbacks or 1/128th of them, whichever is larger. > > > > o Unless the larger of them is more than 100 callbacks, in which > > case there is an additional limit of three milliseconds worth > > of them. > > > > Except that if a given CPU ends up with more than 10,000 callbacks > > (rcutree.qhimark), that CPU's blimit is set to 10,000. > > Also, if in the context of a softirq handler (as opposed to ksoftirqd) > that interrupted the idle task with no pending task, the count of > callbacks is ignored and only the 3-millisecond limit counts. In the > context of ksoftirq, the only limit is that which the scheduler chooses > to impose. > > But it sure seems like the ksoftirqd case should also pay attention to > that 3-millisecond limit. I will queue a patch to that effect, and maybe > Eric Dumazet will show me the error of my ways.
Just to be sure - have you seen Peter's patches?
git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git core/softirq
I think it feeds the time limit to the callback from softirq, so the local 3ms is no more?
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