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SCSI currently uses an atomic variable to track queue depth for each attached device. The queue depth depends on many factors such as transport type and device implementation. In addition, the SCSI device queue depth is not a static entity but changes over time as a result of congestion management. While blk-mq currently tracks queue depth for each hctx, it can't easily be changed to accommodate the SCSI per-device requirement. The current approach of using an atomic variable doesn't scale well when there are lots of CPU cores and the disk is very fast. IOPS can be substantially impacted by the atomic in the hot path. Replace the atomic variable sdev->device_busy with an sbitmap for tracking the SCSI device queue depth. It has been observed that IOPS is improved ~30% by this patchset in the following test: 1) test machine(32 logical CPU cores) Thread(s) per core: 2 Core(s) per socket: 8 Socket(s): 2 NUMA node(s): 2 Model name: Intel(R) Xeon(R) Silver 4110 CPU @ 2.10GHz 2) setup scsi_debug: modprobe scsi_debug virtual_gb=128 max_luns=1 submit_queues=32 delay=0 max_queue=256 3) fio script: fio --rw=randread --size=128G --direct=1 --ioengine=libaio --iodepth=2048 \ --numjobs=32 --bs=4k --group_reporting=1 --group_reporting=1 --runtime=60 \ --loops=10000 --name=job1 --filename=/dev/sdN [mkp: fix device_busy reference in mpt3sas] Link: https://lore.kernel.org/r/20210122023317.687987-14-ming.lei@redhat.com Link: https://lore.kernel.org/linux-block/20200119071432.18558-6-ming.lei@redhat.com/ Cc: Omar Sandoval <osandov@fb.com> Cc: Kashyap Desai <kashyap.desai@broadcom.com> Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com> Cc: Ewan D. Milne <emilne@redhat.com> Cc: Hannes Reinecke <hare@suse.de> Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com> Reviewed-by: Hannes Reinecke <hare@suse.de> Signed-off-by: Ming Lei <ming.lei@redhat.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.