Re: [PATCH RFC] Block/blk-wbt: do not let background writes block sync writes

On 08/25/2017 06:14 PM, Liu Bo wrote:

While using blk-wbt, sometimes sync writes are blocked by background
writes, for example,

a) a background write reaches the (background) limit returned by
get_limit(), so it's added into the rqw->wait (non kswapd's rqw in
this case) and goes to sleep.

b) then a sync write gets queued and goes to sleep when finding that
waitqueue_active() returns true and someone else is already on the
waiting list.

Thus, the sync write will get its rq after the background write
getting rq.

With this, only background writes will check waitqueue's status and
sync writes will only be throttled by the (max) limit returned by
get_limit().

Curious how you ran into this. Is this just speculation, or is it
observed behavior?

I don't think the patch makes sense, conceptually. If you end up in
may_queue(), you are (by definition) in a class of writes that
should be throttled. Skipping the wait line would be unfair.

One modification would be to check the depth, as we assign
different allowances to different types of writes. If the sync
write isn't above it's limit, we should not sleep.

-- 
Jens Axboe