On Fri, 2010-09-03 at 15:27 -0700, Vasu Dev wrote:
> Currently fc_queuecommand drops this lock very early on and then re-acquires
> this lock just before return, this re-acquired lock gets dropped immediately
> by its fast path caller scsi_dispatch_cmd, this re-acquire and then immediate
> drop on each IO hits performance especially with small size IOs on multi-core
> systems, this hit is significant about 25% with 512 bytes size IOs.
>
> This lock is not needed in fc_queuecommand and calling fc_queuecommand
> without this lock held removes above described performance hit.
>
> So this patch adds unlocked_qcmds flag to drop host_lock before
> calling only fc_queuecommand and removes re-acquire & then drop
> for each IO. Added flag, drops this lock only if LLD wants such
> as fcoe.ko does here for fc_queuecommand. This change won't affect
> any existing LLD since added unlocked_qcmds flag will be zero
> in those cases and their host lock uses would effectively remain
> same after this patch.
>
> As per Mike and Nab comments, restored host lock around SHOST_DEL state
> in scsi_dispatch_cmd.
>
> Signed-off-by: Vasu Dev <vasu.dev@xxxxxxxxx>
> ---
> drivers/scsi/fcoe/fcoe.c | 1 +
> drivers/scsi/libfc/fc_fcp.c | 14 +++++---------
> drivers/scsi/scsi.c | 7 ++++++-
> include/scsi/scsi_host.h | 3 +++
> 4 files changed, 15 insertions(+), 10 deletions(-)
>
<SNIP>
> index b7bdecb..1814c51 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -636,6 +636,9 @@ struct Scsi_Host {
> /* Asynchronous scan in progress */
> unsigned async_scan:1;
>
> + /* call queuecommand without Scsi_Host lock held */
> + unsigned unlocked_qcmds:1;
> +
I am not sure why this structure member (or any of the other ones) need
to be using :1 at this point, considering drivers/scsi/hosts.c:scsi_host_alloc()
is doing a kzalloc() on struct Scsi_Host to begin with..
Other than that minor nit, I have merged this patch into branch at
lio-core-2.6.git/scsi-unlocked-qcmds available here:
git://git.kernel.org/pub/scm/linux/kernel/git/nab/lio-core-2.6.git scsi-unlocked-qcmds
and via gitweb here:
http://git.kernel.org/?p=linux/kernel/git/nab/lio-core-2.6.git;a=shortlog;h=refs/heads/scsi-unlocked_qcmds
I will have a look at running some modified TCM_Loop and Linux/iSCSI Initiators tests
using with SHT->unlocked-qcmds=1 in the very near future on this code. 8-)
Many thanks again for your very worthwhile efforts Vasu!
Best,
--nab
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