* [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()
[not found] <20221122010408.GA3799268@paulmck-ThinkPad-P17-Gen-1>
@ 2022-11-22 1:04 ` Paul E. McKenney
2022-11-26 0:11 ` Bart Van Assche
2022-11-26 2:42 ` Martin K. Petersen
0 siblings, 2 replies; 4+ messages in thread
From: Paul E. McKenney @ 2022-11-22 1:04 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, kernel-team, rostedt, Uladzislau Rezki,
Joel Fernandes, James E.J. Bottomley, Martin K. Petersen,
linux-scsi, Paul E . McKenney
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(-)
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);
}
/**
--
2.31.1.189.g2e36527f23
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()
2022-11-22 1:04 ` [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu() Paul E. McKenney
@ 2022-11-26 0:11 ` Bart Van Assche
2022-11-28 21:39 ` Paul E. McKenney
2022-11-26 2:42 ` Martin K. Petersen
1 sibling, 1 reply; 4+ messages in thread
From: Bart Van Assche @ 2022-11-26 0:11 UTC (permalink / raw)
To: Paul E. McKenney, rcu
Cc: linux-kernel, kernel-team, rostedt, Uladzislau Rezki,
Joel Fernandes, James E.J. Bottomley, Martin K. Petersen,
linux-scsi
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>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()
2022-11-22 1:04 ` [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu() Paul E. McKenney
2022-11-26 0:11 ` Bart Van Assche
@ 2022-11-26 2:42 ` Martin K. Petersen
1 sibling, 0 replies; 4+ messages in thread
From: Martin K. Petersen @ 2022-11-26 2:42 UTC (permalink / raw)
To: Paul E. McKenney
Cc: rcu, linux-kernel, kernel-team, rostedt, Uladzislau Rezki,
Joel Fernandes, James E.J. Bottomley, Martin K. Petersen,
linux-scsi
Paul,
> Therefore, make scsi_eh_scmd_add() use call_rcu_flush() in order to
> revert to the old behavior.
[...]
> 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);
> }
>
> /**
OK with me.
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
--
Martin K. Petersen Oracle Linux Engineering
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()
2022-11-26 0:11 ` Bart Van Assche
@ 2022-11-28 21:39 ` Paul E. McKenney
0 siblings, 0 replies; 4+ messages in thread
From: Paul E. McKenney @ 2022-11-28 21:39 UTC (permalink / raw)
To: Bart Van Assche
Cc: rcu, linux-kernel, kernel-team, rostedt, Uladzislau Rezki,
Joel Fernandes, James E.J. Bottomley, Martin K. Petersen,
linux-scsi
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
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-11-22 1:04 ` [PATCH v2 rcu 12/16] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu() Paul E. McKenney
2022-11-26 0:11 ` Bart Van Assche
2022-11-28 21:39 ` Paul E. McKenney
2022-11-26 2:42 ` Martin K. Petersen
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