[v2,rcu,12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()

From: Uladzislau Rezki <urezki@gmail.com>

Earlier commits in this series allow battery-powered systems to build
their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option.
This Kconfig option causes call_rcu() to delay its callbacks in order
to batch them.  This means that a given RCU grace period covers more
callbacks, thus reducing the number of grace periods, in turn reducing
the amount of energy consumed, which increases battery lifetime which
can be a very good thing.  This is not a subtle effect: In some important
use cases, the battery lifetime is increased by more than 10%.

This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.

Delaying callbacks is normally not a problem because most callbacks do
nothing but free memory.  If the system is short on memory, a shrinker
will kick all currently queued lazy callbacks out of their laziness,
thus freeing their memory in short order.  Similarly, the rcu_barrier()
function, which blocks until all currently queued callbacks are invoked,
will also kick lazy callbacks, thus enabling rcu_barrier() to complete
in a timely manner.

However, there are some cases where laziness is not a good option.
For example, synchronize_rcu() invokes call_rcu(), and blocks until
the newly queued callback is invoked.  It would not be a good for
synchronize_rcu() to block for ten seconds, even on an idle system.
Therefore, synchronize_rcu() invokes call_rcu_flush() instead of
call_rcu().  The arrival of a non-lazy call_rcu_flush() callback on a
given CPU kicks any lazy callbacks that might be already queued on that
CPU.  After all, if there is going to be a grace period, all callbacks
might as well get full benefit from it.

Yes, this could be done the other way around by creating a
call_rcu_lazy(), but earlier experience with this approach and
feedback at the 2022 Linux Plumbers Conference shifted the approach
to call_rcu() being lazy with call_rcu_flush() for the few places
where laziness is inappropriate.

And another call_rcu() instance that cannot be lazy is the one in the
scsi_eh_scmd_add() function.  Leaving this instance lazy results in
unacceptably slow boot times.

Therefore, make scsi_eh_scmd_add() use call_rcu_flush() in order to
revert to the old behavior.

Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: <linux-scsi@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 drivers/scsi/scsi_error.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Fri, Nov 25, 2022 at 04:11:26PM -0800, Bart Van Assche wrote:
> On 11/21/22 17:04, Paul E. McKenney wrote:
> > diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> > index 6995c89792300..634672e67c81f 100644
> > --- a/drivers/scsi/scsi_error.c
> > +++ b/drivers/scsi/scsi_error.c
> > @@ -312,7 +312,7 @@ void scsi_eh_scmd_add(struct scsi_cmnd *scmd)
> >   	 * Ensure that all tasks observe the host state change before the
> >   	 * host_failed change.
> >   	 */
> > -	call_rcu(&scmd->rcu, scsi_eh_inc_host_failed);
> > +	call_rcu_flush(&scmd->rcu, scsi_eh_inc_host_failed);
> >   }
> 
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>

Thank you both, I will apply these on the next rebase.

							Thanx, Paul