Re: [patch 1/2]percpu_ida: fix a live lock

[Kent Overstreet]

On Mon, Feb 10, 2014 at 09:26:15AM -0700, Jens Axboe wrote:
> 
> 
> On 02/10/2014 03:32 AM, Christoph Hellwig wrote:
> >On Sun, Feb 09, 2014 at 04:50:07PM +0100, Alexander Gordeev wrote:
> >>Yeah, that was my first thought when I posted "percpu_ida: Allow variable
> >>maximum number of cached tags" patch some few months ago. But I am back-
> >>pedalling as it does not appear solves the fundamental problem - what is the
> >>best threshold?
> >>
> >>May be we can walk off with a per-cpu timeout that flushes batch nr of tags
> >>from local caches to the pool? Each local allocation would restart the timer,
> >>but once allocation requests stopped coming on a CPU the tags would not gather
> >>dust in local caches.
> >
> >We'll defintively need a fix to be able to allow the whole tag space.
> 
> Certainly. The current situation of effectively only allowing half
> the tags (if spread) is pretty crappy with (by far) most hardware.
> 
> >For large numbers of tags per device the flush might work, but for
> >devices with low number of tags we need something more efficient.  The
> >case of less tags than CPUs isn't that unusual either and we probably
> >want to switch to an allocator without per cpu allocations for them to
> >avoid all this.  E.g. for many ATA devices we just have a single tag,
> >and many scsi drivers also only want single digit outstanding commands
> >per LUN.
> 
> Even for cases where you have as many (or more) CPUs than tags,
> per-cpu allocation is not necessarily a bad idea. It's a rare case
> where you have all the CPUs touching the device at the same time,
> after all.

<just back from Switzerland, probably forgetting some of where I left off>

You do still need to have enough tags to shard across the number of cpus
_currently_ touching the device. I think I'm with Christoph here, I'm not sure
how percpu tag allocation would be helpful when we have single digits/low double
digits of tags available.

I would expect that in that case we're better off with just a well implemented
atomic bit vector and waitlist. However, I don't know where the crossover point
is and I think Jens has done by far the most and most relevant benchmarking
here.

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.
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