linux-kernel - Re: [PATCH rfc] workqueue: honour cond

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Date:   Fri, 27 Nov 2020 10:44:14 +1100
From:   NeilBrown <neilb@...e.de>
To:     Hillf Danton <hdanton@...a.com>
Cc:     TJ <tj@...nel.org>, Trond Myklebust <trondmy@...merspace.com>,
        PeterZ <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH rfc] workqueue: honour cond_resched() more effectively.

On Thu, Nov 26 2020, Hillf Danton wrote:

> On Fri, 20 Nov 2020 15:33:27 +1100 NeilBrown wrote:
>>
>>My first idea was to add WQ_CPU_INTENSIVE to the nfsiod workqueue, but
>>Trond wondered what was special about NFS.  Many filesystems call iput
>>from a workqueue, so finding a solution that helps them all is best.
>
> In terms of iput, I think we can splice WQ_CPU_INTENSIVE to
> WQ_MEM_RECLAIM.

I'm actually starting to think that WQ_CPU_INTENSIVE is a mistake.  If
you really have cpu-intensive work, you should be using WQ_UNBOUND.

It is possible that there might be work that is CPU intensive and which
must be run on a particular CPU - such as clearing out per-cpu lists of
recently freed slab allocations.  But I don't WQ_CPU_INTENSIVE is currently
used that way.

I cannot find *any* users of WQ_CPU_INTENSIVE which call cond_resched()
in the relevant work items.  And if the code doesn't call cond_resched()
(or similar), then it isn't really CPU-intensive.

>
>>I then suggested getting cond_resched() to do something more useful when
>>called by a worker.  PeterZ didn't like the overhead.
>>
>>Also, TJ seemed to be against auto-adjusting for cpu-intensive code,
>>preferring the right sort of workqueue to be chosen up front.
>
> Actually WQ_EVENTS_LONG sounds better than WQ_CPU_INTENSIVE, given that
> we have two events WQs with the same attr.

There is no WQ_EVENTS_LONG

>
> 	system_wq = alloc_workqueue("events", 0, 0);
> 	system_long_wq = alloc_workqueue("events_long", 0, 0);
>
> Then what are the boundaries we can draw in between WQ_MEM_RECLAIM,
> WQ_CPU_INTENSIVE and WQ_EVENTS_LONG?

I think system_long_wq is a design flaw.
Some code (mistakenly) schedules work on system_wq, calls
flush_workqueue(system_wq)) and expects that to complete reasonably quickly.
To ensure this can work, system_long_wq was created and work items that
might take a long time are encouraged to be run there.
Instead, the mistaken code should create its own work queue, schedule
work on that, and flush that queue.

Thanks,
NeilBrown

>
>
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -4261,6 +4261,9 @@ struct workqueue_struct *alloc_workqueue
>  	if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
>  		flags |= WQ_UNBOUND;
>  
> +	if (flags & (WQ_MEM_RECLAIM | WQ_UNBOUND) == WQ_MEM_RECLAIM)
> +		flags |= WQ_CPU_INTENSIVE;
> +
>  	/* allocate wq and format name */
>  	if (flags & WQ_UNBOUND)
>  		tbl_size = nr_node_ids * sizeof(wq->numa_pwq_tbl[0]);

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