On Dec 8, 2018, at 10:51 PM, Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxx> > > 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.
