linux-kernel - Re: [PATCH 2/3] sbitmap: ammortize cost of clearing bits

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Date:   Sat, 8 Dec 2018 23:19:59 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-block@...r.kernel.org, osandov@...ndov.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sbitmap: ammortize cost of clearing bits

On Dec 8, 2018, at 10:51 PM, Guenter Roeck <linux@...ck-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@...nel.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.