Re: [PATCH] sched/fair: no sync wakeup from interrupt context

Libo Chen
2022-07-13T21:37:50Z

On 7/13/22 13:51, Tim Chen wrote:
> 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.
Yeah, we should rewrite wake_affine_weight() so that it compares average
loads of two nodes
instead of two rq loads.

Libo

>>> 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
>
>