linux-kernel - Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers accept task state bitmask

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Message-ID: <20140120113415.GE30183@twins.programming.kicks-ass.net>
Date:	Mon, 20 Jan 2014 12:34:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	target-devel <target-devel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kent Overstreet <kmo@...erainc.com>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers
 accept task state bitmask

On Mon, Jan 20, 2014 at 03:44:44AM +0000, Nicholas A. Bellinger wrote:
> From: Kent Overstreet <kmo@...erainc.com>
> 
> 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.

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

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

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

Help?
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