linux-kernel - RE: Question about ufs

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Date:   Tue, 14 Sep 2021 15:45:17 +0900
From:   "Kiwoong Kim" <kwmad.kim@...sung.com>
To:     "'Avri Altman'" <Avri.Altman@....com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <alim.akhtar@...sung.com>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <beanhuo@...ron.com>,
        <cang@...eaurora.org>, <adrian.hunter@...el.com>,
        <sc.suh@...sung.com>, <hy50.seo@...sung.com>,
        <sh425.lee@...sung.com>, <bhoon95.kim@...sung.com>
Subject: RE: Question about ufs_bsg

> Hi,
> 
> > Hi,
> >
> > ufs_bsg was introduced nearly three years ago and it allocates its own
> > request queue.
> > I faced a sytmpom with this and want to ask something about it.
> >
> > That is, sometimes queue depth for ufs is limited to half of the its
> > maximum value even in a situation with many IO requests from
> > filesystem.
> This is interesting indeed. Before going further with investigating this,

Hi. What I first intended is not ufs_bsg but as you might already know, it also allocated its own request queue.
In that point, we can imagine it could be the same situation.

> Could you share some more details on your setup:
> The bsg node it creates was originally meant to convey a single query
> request via SG_IO ioctl, Which is blocking.
>  - How do you create many IO requests queueing on that request queue?

I used some benchmarks, such tiobench or Androbench that could make heavy IO scenarios.

>  - command upiu is not implemented, are all those IOs are query requests?

What I've seen is just one query and many scsi commands.

> 
> > It turned out that it only occurs when a query is being processed at
> > the same time.
> > Regarding my tracing, when the query process starts, users for the
> > hctx that represents a ufs host increase to two and with this, some
> > pathes calling 'hctx_may_queue'
> > function in blk-mq seems to throttle dispatches, technically with 16
> > because the number of ufs slots (32 in my case) is dividend by two
> > (users).
> >
> > I found that it happened when a query for write booster is processed
> > because write booster only turns on in some conditions in my base that
> > is different from kernel mainline. But when an exceptional event or
> > others that could lead to a query occurs, it can happen even in
> > mainline.
> >
> > I think the throttling is a little bit excessive, so the question: is
> > there any way to assign queue depth per user on an asymmetric basis?
> >
> > Thanks.
> > Kiwoong Kim
> >