linux-kernel - Re: [patch 1/2]percpu

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Date:	Tue, 11 Feb 2014 06:42:40 -0800
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	Alexander Gordeev <agordeev@...hat.com>,
	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [patch 1/2]percpu_ida: fix a live lock

On Tue, 2014-02-11 at 01:12 -0800, Christoph Hellwig wrote:
> On Mon, Feb 10, 2014 at 04:06:27PM -0700, Jens Axboe wrote:
> > For the common case, I'd assume that anywhere between 31..256 tags
> > is "normal". That's where the majority of devices will end up being,
> > largely. So single digits would be an anomaly.
> 
> Unfortunately that's not true in SCSI land, where most driver do per-lun
> tagging, and the the cmd_per_lun values are very low and very often
> single digits, as a simple grep for cmd_per_lun will tell.

Remember we do shared (all queue) tags on qla, aic and a few other
drivers (it's actually the mailbox slot tag for the HBA).

>   Now it might
> be that the tag space actually is much bigger in the hardware and the
> driver authors for some reason want to limit the number of outstanding
> commands, but the interface to the drivers doesn't allow them to express
> such a difference at the moment.

Tag space is dependent on SCSI protocol.  It's 256 for SPI, 65536 for
SAS and I'm not sure for FCP.

> > >How about we just make the number of tags that are allowed to be stranded an
> > >explicit parameter (somehow) - then it can be up to device drivers to do
> > >something sensible with it. Half is probably an ideal default for devices where
> > >that works, but this way more constrained devices will be able to futz with it
> > >however they want.
> > 
> > I don't think we should involve device drivers in this, that's
> > punting a complicated issue to someone who likely has little idea
> > what to do about it. This needs to be handled sensibly in the core,
> > not in a device driver. If we can't come up with a sensible
> > algorithm to handle this, how can we expect someone writing a device
> > driver to do so?
> 
> Agreed, punting this to the drivers is a bad idea.  But at least
> exposing variable for the allowed tag space and allowed outstanding
> commands to be able to make a smarter decision might be a good idea.  On
> the other hand this will require us to count the outstanding commands
> again, introducing more cachelines touched than nessecary.  To make
> things worse for complex topologies like SCSI we might have to limit the
> outstanding commands at up to three levels in the hierarchy.

The list seems to be missing prior context but a few SPI drivers use the
clock algorithm for tag starvation in the driver.  The NCR ones are the
ones I know about: tag allocation is the hands of a clock sweeping
around (one for last tag and one for last outstanding tag).  The hands
are never allowed to cross, so if a tag gets starved the hands try to
cross and the driver stops issuing until the missing tag returns.  Tag
starvation used to be a known problem for Parallel devices; I haven't
seen much in the way of tag starvation algorithms for other types of
devices, so I assume the problem went away.

James


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