linux-kernel - RE: Question about ufs

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Date:   Tue, 14 Sep 2021 06:39:00 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     Kiwoong Kim <kwmad.kim@...sung.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "beanhuo@...ron.com" <beanhuo@...ron.com>,
        "cang@...eaurora.org" <cang@...eaurora.org>,
        "adrian.hunter@...el.com" <adrian.hunter@...el.com>,
        "sc.suh@...sung.com" <sc.suh@...sung.com>,
        "hy50.seo@...sung.com" <hy50.seo@...sung.com>,
        "sh425.lee@...sung.com" <sh425.lee@...sung.com>,
        "bhoon95.kim@...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,
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?
 - command upiu is not implemented, are all those IOs are query requests?

Thanks,
Avri

> 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
>