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From: Jens Axboe <axboe@kernel.dk>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-block@vger.kernel.org, osandov@osandov.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] sbitmap: ammortize cost of clearing bits
Date: Sat, 8 Dec 2018 23:19:59 -0700	[thread overview]
Message-ID: <6AAE999B-AB08-46D0-9DF8-9CED32E6141B@kernel.dk> (raw)
In-Reply-To: <20181209055146.GA17190@roeck-us.net>

On Dec 8, 2018, at 10:51 PM, Guenter Roeck <linux@roeck-us.net> wrote:
> 
> Hi,
> 
>> On Thu, Nov 29, 2018 at 06:12:33PM -0700, Jens Axboe wrote:
>> sbitmap maintains a set of words that we use to set and clear bits, with
>> each bit representing a tag for blk-mq. Even though we spread the bits
>> out and maintain a hint cache, one particular bit allocated will end up
>> being cleared in the exact same spot.
>> 
>> This introduces batched clearing of bits. Instead of clearing a given
>> bit, the same bit is set in a cleared/free mask instead. If we fail
>> allocating a bit from a given word, then we check the free mask, and
>> batch move those cleared bits at that time. This trades 64 atomic bitops
>> for 2 cmpxchg().
>> 
>> In a threaded poll test case, half the overhead of getting and clearing
>> tags is removed with this change. On another poll test case with a
>> single thread, performance is unchanged.
>> 
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> This patch results in irq lock inversion warnings when trying to boot
> from usb drives. This was observed with qemu boots of aarch64, x86, and
> x86_64 images.

This one is a false positive, was already reported last week. Just need to add some lockdep annotation for it. 



  reply	other threads:[~2018-12-09  6:20 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20181130011234.32674-1-axboe@kernel.dk>
2018-11-30  1:12 ` [PATCH 1/3] sbitmap: ensure that sbitmap maps are properly aligned Jens Axboe
2018-11-30  1:12 ` [PATCH 2/3] sbitmap: ammortize cost of clearing bits Jens Axboe
2018-12-09  5:51   ` Guenter Roeck
2018-12-09  6:19     ` Jens Axboe [this message]
2018-11-30  1:12 ` [PATCH 3/3] sbitmap: optimize wakeup check Jens Axboe
2018-11-30  2:09 ` Jens Axboe

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