* [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() 2022-11-26 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
* 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
end of thread, other threads:[~2022-11-28 21:39 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- [not found] <20221122010408.GA3799268@paulmck-ThinkPad-P17-Gen-1> 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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