Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callersaccept task state bitmask

[Nicholas A. Bellinger]

On Mon, 2014-01-20 at 12:34 +0100, Peter Zijlstra wrote:
> On Mon, Jan 20, 2014 at 03:44:44AM +0000, Nicholas A. Bellinger wrote:
> > From: Kent Overstreet <kmo@xxxxxxxxxxxxx>
> > 
> > This patch changes percpu_ida_alloc() + callers to accept task state
> > bitmask for prepare_to_wait() for code like target/iscsi that needs
> > it for interruptible sleep, that is provided in a subsequent patch.
> > 
> > It now expects TASK_UNINTERRUPTIBLE when the caller is able to sleep
> > waiting for a new tag, or TASK_RUNNING when the caller cannot sleep,
> > and is forced to return a negative value when no tags are available.
> > 
> > v2 changes:
> >   - Include blk-mq + tcm_fc + vhost/scsi + target/iscsi changes
> >   - Drop signal_pending_state() call
> 
> Urgh, you made me look at percpu_ida... steal_tags() does a
> for_each_cpus() with IRQs disabled. This mean you'll disable IRQs for
> multiple ticks on SGI class hardware. That is a _very_ long time indeed.
> 

So given the performance penalties involved in the steal tag slow path,
consumers should typically be pre-allocating a larger number of
percpu_ida tags than necessary to (ideally) avoid this logic completely.

> Then there's alloc_global_tags() vs alloc_local_tags(), one gets an
> actual tag, while the other only moves tags about -- semantic mismatch.
> 

How about just in-lining alloc_global_tags() into percpu_ida_alloc()..?

> I do not get the comment near prepare to wait -- why does it matter if
> percpu_ida_free() flips a cpus_have_tags bit?
> 

Mmm, not sure on that one.

> Given I don't understand this comment, its hard for me to properly
> review the proposed patch series.
> 

Kent..?

--nab

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