Messages in this thread | | | Subject | Re: [PATCH] sched/fair: no sync wakeup from interrupt context | From | Tim Chen <> | Date | Wed, 13 Jul 2022 13:51:05 -0700 |
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On Wed, 2022-07-13 at 12:17 -0700, Libo Chen wrote: > > > On 7/13/22 09:40, Tim Chen wrote: > > On Mon, 2022-07-11 at 15:47 -0700, Libo Chen wrote: > > > Barry Song first pointed out that replacing sync wakeup with regular wakeup > > > seems to reduce overeager wakeup pulling and shows noticeable performance > > > improvement.[1] > > > > > > This patch argues that allowing sync for wakeups from interrupt context > > > is a bug and fixing it can improve performance even when irq/softirq is > > > evenly spread out. > > > > > > For wakeups from ISR, the waking CPU is just the CPU of ISR and the so-called > > > waker can be any random task that happens to be running on that CPU when the > > > interrupt comes in. This is completely different from other wakups where the > > > task running on the waking CPU is the actual waker. For example, two tasks > > > communicate through a pipe or mutiple tasks access the same critical section, > > > etc. This difference is important because with sync we assume the waker will > > > get off the runqueue and go to sleep immedately after the wakeup. The > > > assumption is built into wake_affine() where it discounts the waker's presence > > > from the runqueue when sync is true. The random waker from interrupts bears no > > > relation to the wakee and don't usually go to sleep immediately afterwards > > > unless wakeup granularity is reached. Plus the scheduler no longer enforces the > > > preepmtion of waker for sync wakeup as it used to before > > > patch f2e74eeac03ffb7 ("sched: Remove WAKEUP_SYNC feature"). Enforcing sync > > > wakeup preemption for wakeups from interrupt contexts doesn't seem to be > > > appropriate too but at least sync wakeup will do what it's supposed to do. > > > > Will there be scenarios where you do want the task being woken up be pulled > > to the CPU where the interrupt happened, as the data that needs to be accessed is > > on local CPU/NUMA that interrupt happened? For example, interrupt associated with network > > packets received. Sync still seems desirable, at least if there is no task currently > > running on the CPU where interrupt happened. So maybe we should have some consideration > > of the load on the CPU/NUMA before deciding whether we should do sync wake for such > > interrupt. > > > There are only two places where sync wakeup matters: wake_affine_idle() and wake_affine_weight(). > In wake_affine_idle(), it considers pulling if there is one runnable on the waking CPU because > of the belief that this runnable will voluntarily get off the runqueue. In wake_affine_weight(), > it basically takes off the waker's load again assuming the waker goes to sleep after the wakeup. > My argument is that this assumption doesn't really hold for wakeups from the interrupt contexts > when the waking CPU is non-idle. Wakeups from task context? sure, it seems to be a reasonable > assumption.
I agree with you that the the sync case load computation for wake_affine_idle() and wake_affine_weight() is incorrect when waking a task from the interrupt context. In this light, your proposal makes sense.
> For your idle case, I totally agree but I don't think having sync or not will actually > have any impacts here giving what the code does. Real impact comes from Mel's patch 7332dec055f2457c3 > which makes it less likely to pull tasks when the waking CPU is idle. I believe we should consider > reverting 7332dec055f2 because a significant RDS latency regression has been spotted recently on our > system due to this patch.
The commit 7332dec055f2 prevented cross NUMA node pulling. I think if the waking CPU's NUMA node's average load is less than the prev CPU's NUMA node, this cross NUMA node pull could be allowed for better load distribution.
> > > > Can you provide some further insights on why pgebench is helped at low load > > case? Is it because the woken tasks tend to stay put and not get moved around with interrupts > > and maintain cache warmth? > Yes, and for read-only database workloads, the cache (whether it's incoming packet or not) on the interrupt > CPU isn't as performance critical as cache from its previous CPU where the db task run to process data. > To give you an example, consider a db client/server case, a client sends a request for a select query > through the network, the server accepts the query request and does all the heavy lifting and sends the result > back. For the server, the incoming packet is just a line of query whereas the CPU and its L3 db process previously > on has all the warm db caches, pulling it away from them is a crime :) This may seem to be a little contradictory > to what I said earlier about the idle case and Mel's patch, but ¯\_(ツ)_/¯ it's hard to make all the workloads out > there happy. Anyway like I said earlier, this patch doesn't affect the idle case > > At higher load, sync in wake_affine_idle() doesn't really matter because the waking CPU could easily have more than > 1 runnable tasks. Sync in wake_affine_weight() also doesn't matter much as both sides have work to do, and cache > benefit of not pulling decreases simply because there are a lot more db processes under the same L3, they can compete > for the same cachelines. > > Hope my explanation helps!
Yes, that makes sense.
Tim
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