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

[Martin Wilck <martin.wilck@suse.com>]

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