linux-kernel - Re: [PATCH 11/17 SQUASH and replace commit message] lib: add light-weight queuing mechanism.

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Date:   Fri, 15 Sep 2023 15:54:58 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Neil Brown <neilb@...e.de>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        Liam Howlett <liam.howlett@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Gow <davidgow@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/17 SQUASH and replace commit message] lib: add
 light-weight queuing mechanism.



> On Sep 14, 2023, at 10:22 PM, NeilBrown <neilb@...e.de> wrote:
> 
> 
> lwq is a FIFO single-linked queue that only requires a spinlock
> for dequeueing, which happens in process context.  Enqueueing is atomic
> with no spinlock and can happen in any context.
> 
> This is particularly useful when work items are queued from BH or IRQ
> context, and when they are handled one at a time by dedicated threads.
> 
> Avoiding any locking when enqueueing means there is no need to disable
> BH or interrupts, which is generally best avoided (particularly when
> there are any RT tasks on the machine).
> 
> This solution is superior to using "list_head" links because we need
> half as many pointers in the data structures, and because list_head
> lists would need locking to add items to the queue.
> 
> This solution is superior to a bespoke solution as all locking and
> container_of casting is integrated, so the interface is simple.
> 
> Despite the similar name, this solution meets a distinctly different
> need to kfifo.  kfifo provides a fixed sized circular buffer to which
> data can be added at one end and removed at the other, and does not
> provide any locking.  lwq does not have any size limit and works with
> data structures (objects?) rather than data (bytes).
> 
> A unit test for basic functionality, which runs at boot time, is included.
> 
> Signed-off-by: NeilBrown <neilb@...e.de>

Thank you for all of this work, Neil. The metrics in the
cover letter speak for themselves. I've applied the series
to nfsd-next and squashed this patch in.


> ---
> include/linux/lwq.h | 4 ++++
> lib/lwq.c           | 4 ++++
> 2 files changed, 8 insertions(+)
> 
> diff --git a/include/linux/lwq.h b/include/linux/lwq.h
> index 52b9c81b493a..c4148fe1cf72 100644
> --- a/include/linux/lwq.h
> +++ b/include/linux/lwq.h
> @@ -7,6 +7,10 @@
>  *
>  * Entries can be enqueued from any context with no locking.
>  * Entries can be dequeued from process context with integrated locking.
> + *
> + * This is particularly suitable when work items are queued in
> + * BH or IRQ context, and where work items are handled one at a time
> + * by dedicated threads.
>  */
> #include <linux/container_of.h>
> #include <linux/spinlock.h>
> diff --git a/lib/lwq.c b/lib/lwq.c
> index 7fe6c7125357..eb8324225309 100644
> --- a/lib/lwq.c
> +++ b/lib/lwq.c
> @@ -8,6 +8,10 @@
>  * Entries are dequeued using a spinlock to protect against
>  * multiple access.  The llist is staged in reverse order, and refreshed
>  * from the llist when it exhausts.
> + * 
> + * This is particularly suitable when work items are queued in
> + * BH or IRQ context, and where work items are handled one at a time
> + * by dedicated threads.
>  */
> #include <linux/rcupdate.h>
> #include <linux/lwq.h>
> -- 
> 2.42.0
> 

--
Chuck Lever