Re: [PATCH] dm-verity: support block number limits for different ioprio classes

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Mar 26, 2025 at 10:09:22AM +0800, LongPing Wei wrote:
> Hi, Eric
> 
> > If the CPU has been in softirq context for too long, then stop
> > choosing softirq context and instead fall back to the traditional
> > workqueue.  This could help address objections about increased use of >
> softirq context.
> 
> Could you share me some example about this?
> 

The objection that keeps getting raised to doing more work in softirq context is
that the latency of real-time tasks, such as audio tasks, may increase due to
softirqs having a higher priority than all tasks.  Causing audio skipping, etc.

The logic in handle_softirqs() in kernel/softirq.c actually goes a ways towards
addressing this already: it processes softirqs for at most 2 ms or until
rescheduling of the interrupted task gets requested, before deferring them to
ksoftirqd which runs at normal task priority.

The gaps that I see are (a) 2 ms is longer than desired, and (b) the limit does
not apply to *individual* softirqs.  Looking at (b), observe that the block
softirq (blk_done_softirq()) completes a list of I/O requests.  If there are a
lot of requests in that list, it could theoretically take more than the 2 ms
limit that kernel/softirq.c is meant to enforce (which is already too long).

So the objection would be that, even with dm-verity choosing to do in-line
verification only for 4 KiB requests which take only a few microseconds each, a
lot of requests could still add up to cause a long time to be spent in a single
softirq context.  (At least in theory.  AFAIK no one has confirmed that this is
actually a problem in practice with the 4 KiB limit applied.  But this is the
objection that I keep hearing whenever anyone suggests doing more in softirqs.)

But if dm-verity could detect this case happening and start deferring
verification work to task context, that should mostly address this concern, IMO.

One idea which *might* get us most of the way there pretty easily would be to do
the in-line verification only when need_resched() returns false.

- Eric