[PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Martin Wilck]

For multipath devices, nvme_update_ns_info() needs to freeze both
the queue of the path and the queue of the multipath device. For
both operations, it waits for one RCU grace period to pass, ~25ms
on my test system. By calling blk_freeze_queue_start() for the
multipath queue early, we avoid waiting twice; tests using ftrace
have shown that the second blk_mq_freeze_queue_wait() call finishes
in just a few microseconds. The path queue is unfrozen before
calling blk_mq_freeze_queue_wait() on the multipath queue, so that
possibly outstanding IO in the multipath queue can be flushed.

I tested this using the "controller rescan under I/O load" test
I submitted recently [1].

[1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u

Signed-off-by: Martin Wilck <mwilck@suse.com>
---
 drivers/nvme/host/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 33fa01c599ad..e2454398c660 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 	bool unsupported = false;
 	int ret;
 
+	if (nvme_ns_head_multipath(ns->head))
+		blk_freeze_queue_start(ns->head->disk->queue);
+
 	switch (info->ids.csi) {
 	case NVME_CSI_ZNS:
 		if (!IS_ENABLED(CONFIG_BLK_DEV_ZONED)) {
@@ -2254,7 +2257,7 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 		struct queue_limits *ns_lim = &ns->disk->queue->limits;
 		struct queue_limits lim;
 
-		blk_mq_freeze_queue(ns->head->disk->queue);
+		blk_mq_freeze_queue_wait(ns->head->disk->queue);
 		/*
 		 * queue_limits mixes values that are the hardware limitations
 		 * for bio splitting with what is the device configuration.
@@ -2288,7 +2291,8 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 		nvme_mpath_revalidate_paths(ns);
 
 		blk_mq_unfreeze_queue(ns->head->disk->queue);
-	}
+	} else if (nvme_ns_head_multipath(ns->head))
+		blk_mq_unfreeze_queue(ns->head->disk->queue);
 
 	return ret;
 }
-- 
2.35.3

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Hannes Reinecke]

On 8/22/24 22:14, Martin Wilck wrote:
> For multipath devices, nvme_update_ns_info() needs to freeze both
> the queue of the path and the queue of the multipath device. For
> both operations, it waits for one RCU grace period to pass, ~25ms
> on my test system. By calling blk_freeze_queue_start() for the
> multipath queue early, we avoid waiting twice; tests using ftrace
> have shown that the second blk_mq_freeze_queue_wait() call finishes
> in just a few microseconds. The path queue is unfrozen before
> calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> possibly outstanding IO in the multipath queue can be flushed.
> 
> I tested this using the "controller rescan under I/O load" test
> I submitted recently [1].
> 
> [1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>   drivers/nvme/host/core.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Daniel Wagner]

On Thu, Aug 22, 2024 at 10:14:13PM GMT, Martin Wilck wrote:
> For multipath devices, nvme_update_ns_info() needs to freeze both
> the queue of the path and the queue of the multipath device. For
> both operations, it waits for one RCU grace period to pass, ~25ms
> on my test system. By calling blk_freeze_queue_start() for the
> multipath queue early, we avoid waiting twice; tests using ftrace
> have shown that the second blk_mq_freeze_queue_wait() call finishes
> in just a few microseconds. The path queue is unfrozen before
> calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> possibly outstanding IO in the multipath queue can be flushed.
> 
> I tested this using the "controller rescan under I/O load" test
> I submitted recently [1].
> 
> [1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>

Reviewed-by: Daniel Wagner <dwagner@suse.de>

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Niklas Cassel]

On Thu, Aug 22, 2024 at 10:14:13PM +0200, Martin Wilck wrote:
> For multipath devices, nvme_update_ns_info() needs to freeze both
> the queue of the path and the queue of the multipath device. For
> both operations, it waits for one RCU grace period to pass, ~25ms
> on my test system. By calling blk_freeze_queue_start() for the
> multipath queue early, we avoid waiting twice; tests using ftrace
> have shown that the second blk_mq_freeze_queue_wait() call finishes
> in just a few microseconds. The path queue is unfrozen before
> calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> possibly outstanding IO in the multipath queue can be flushed.
> 
> I tested this using the "controller rescan under I/O load" test
> I submitted recently [1].
> 
> [1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  drivers/nvme/host/core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 33fa01c599ad..e2454398c660 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
>  	bool unsupported = false;
>  	int ret;
>  
> +	if (nvme_ns_head_multipath(ns->head))
> +		blk_freeze_queue_start(ns->head->disk->queue);
> +

From someone reading this code, it looks quite similar to
nvme_mpath_start_freeze().

Perhaps create a new helper, with proper kdoc, and possibly also add kdoc to
nvme_mpath_start_freeze(), so that a user can easily tell (from the kdoc)
when to use which function.


Kind regards,
Niklas

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Martin Wilck]

On Fri, 2024-08-23 at 15:38 +0200, Niklas Cassel wrote:
> On Thu, Aug 22, 2024 at 10:14:13PM +0200, Martin Wilck wrote:
> > For multipath devices, nvme_update_ns_info() needs to freeze both
> > the queue of the path and the queue of the multipath device. For
> > both operations, it waits for one RCU grace period to pass, ~25ms
> > on my test system. By calling blk_freeze_queue_start() for the
> > multipath queue early, we avoid waiting twice; tests using ftrace
> > have shown that the second blk_mq_freeze_queue_wait() call finishes
> > in just a few microseconds. The path queue is unfrozen before
> > calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> > possibly outstanding IO in the multipath queue can be flushed.
> > 
> > I tested this using the "controller rescan under I/O load" test
> > I submitted recently [1].
> > 
> > [1]
> > https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> > 
> > Signed-off-by: Martin Wilck <mwilck@suse.com>
> > ---
> >  drivers/nvme/host/core.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> > index 33fa01c599ad..e2454398c660 100644
> > --- a/drivers/nvme/host/core.c
> > +++ b/drivers/nvme/host/core.c
> > @@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns
> > *ns, struct nvme_ns_info *info)
> >  	bool unsupported = false;
> >  	int ret;
> >  
> > +	if (nvme_ns_head_multipath(ns->head))
> > +		blk_freeze_queue_start(ns->head->disk->queue);
> > +
> 
> From someone reading this code, it looks quite similar to
> nvme_mpath_start_freeze().

That function takes a struct nvme_subsystem as argument, and walks over
all namespaces in that subsystem, whereas here we're just acting on a
single namespace.

> Perhaps create a new helper, with proper kdoc, and possibly also add
> kdoc to
> nvme_mpath_start_freeze(), so that a user can easily tell (from the
> kdoc)
> when to use which function.

What's the benefit of introducing such a trivial helper, used only in a
single place of the code?
Thanks for the suggestion, but I'd like to see other maintainers'
opinions about this.

Thanks,
Martin

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Niklas Cassel]

On Fri, Aug 23, 2024 at 05:26:56PM +0200, Martin Wilck wrote:
> On Fri, 2024-08-23 at 15:38 +0200, Niklas Cassel wrote:
> > On Thu, Aug 22, 2024 at 10:14:13PM +0200, Martin Wilck wrote:
> > > For multipath devices, nvme_update_ns_info() needs to freeze both
> > > the queue of the path and the queue of the multipath device. For
> > > both operations, it waits for one RCU grace period to pass, ~25ms
> > > on my test system. By calling blk_freeze_queue_start() for the
> > > multipath queue early, we avoid waiting twice; tests using ftrace
> > > have shown that the second blk_mq_freeze_queue_wait() call finishes
> > > in just a few microseconds. The path queue is unfrozen before
> > > calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> > > possibly outstanding IO in the multipath queue can be flushed.
> > > 
> > > I tested this using the "controller rescan under I/O load" test
> > > I submitted recently [1].
> > > 
> > > [1]
> > > https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> > > 
> > > Signed-off-by: Martin Wilck <mwilck@suse.com>
> > > ---
> > >  drivers/nvme/host/core.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> > > index 33fa01c599ad..e2454398c660 100644
> > > --- a/drivers/nvme/host/core.c
> > > +++ b/drivers/nvme/host/core.c
> > > @@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns
> > > *ns, struct nvme_ns_info *info)
> > >  	bool unsupported = false;
> > >  	int ret;
> > >  
> > > +	if (nvme_ns_head_multipath(ns->head))
> > > +		blk_freeze_queue_start(ns->head->disk->queue);
> > > +
> > 
> > From someone reading this code, it looks quite similar to
> > nvme_mpath_start_freeze().
> 
> That function takes a struct nvme_subsystem as argument, and walks over
> all namespaces in that subsystem, whereas here we're just acting on a
> single namespace.
> 
> > Perhaps create a new helper, with proper kdoc, and possibly also add
> > kdoc to
> > nvme_mpath_start_freeze(), so that a user can easily tell (from the
> > kdoc)
> > when to use which function.
> 
> What's the benefit of introducing such a trivial helper, used only in a
> single place of the code?

To make nvme_update_ns_info() easier to read, and to keep the existing code
style of calling nvme_mpath_*() functions unconditionally.

Sure, with my suggestion you would need a mpath_freeze() and mpath_unfreeze()
helper.

But if you add a helper that simply does:
if (nvme_ns_head_multipath(ns->head))
	blk_freeze_queue_start(ns->head->disk->queue);

nvme_update_ns_info() could call the helper unconditionally,
just like how e.g. nvme_passthru_start() calls nvme_mpath_start_freeze()
unconditionally for both multipath and non-multipath NS heads:
https://github.com/torvalds/linux/blob/v6.11-rc4/drivers/nvme/host/core.c#L1205

(And how core.c calls many other nvme_mpath_*() functions unconditionally.)

> Thanks for the suggestion, but I'd like to see other maintainers'
> opinions about this.

Of course, a suggestion is just a suggestion :)


Kind regards,
Niklas

Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Sagi Grimberg]

The patch title should explain what it is fixing, i.e.  the reason of 
its existence.
Perhaps something like:
nvme: shorten multipath ns update duration

On 22/08/2024 23:14, Martin Wilck wrote:
> For multipath devices, nvme_update_ns_info() needs to freeze both
> the queue of the path and the queue of the multipath device. For
> both operations, it waits for one RCU grace period to pass, ~25ms
> on my test system. By calling blk_freeze_queue_start() for the
> multipath queue early, we avoid waiting twice; tests using ftrace
> have shown that the second blk_mq_freeze_queue_wait() call finishes
> in just a few microseconds. The path queue is unfrozen before
> calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> possibly outstanding IO in the multipath queue can be flushed.
>
> I tested this using the "controller rescan under I/O load" test
> I submitted recently [1].
>
> [1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
>
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>   drivers/nvme/host/core.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 33fa01c599ad..e2454398c660 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
>   	bool unsupported = false;
>   	int ret;
>   
> +	if (nvme_ns_head_multipath(ns->head))
> +		blk_freeze_queue_start(ns->head->disk->queue);

Lets add a small comment here.

> +
>   	switch (info->ids.csi) {
>   	case NVME_CSI_ZNS:
>   		if (!IS_ENABLED(CONFIG_BLK_DEV_ZONED)) {
> @@ -2254,7 +2257,7 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
>   		struct queue_limits *ns_lim = &ns->disk->queue->limits;
>   		struct queue_limits lim;
>   
> -		blk_mq_freeze_queue(ns->head->disk->queue);
> +		blk_mq_freeze_queue_wait(ns->head->disk->queue);
>   		/*
>   		 * queue_limits mixes values that are the hardware limitations
>   		 * for bio splitting with what is the device configuration.
> @@ -2288,7 +2291,8 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
>   		nvme_mpath_revalidate_paths(ns);
>   
>   		blk_mq_unfreeze_queue(ns->head->disk->queue);
> -	}
> +	} else if (nvme_ns_head_multipath(ns->head))
> +		blk_mq_unfreeze_queue(ns->head->disk->queue);

Right now if ret!=0 you are unfreezing without waiting, you need to wait 
for the freeze to
complete before unfreezing.

You should restructure the code to make it so that the freeze_start, 
freeze_wait, unfreeze are
paired regardless of the code flows.