From: Peter Zijlstra <peterz@infradead.org>
To: Alex Kogan <alex.kogan@oracle.com>
Cc: linux@armlinux.org.uk, mingo@redhat.com, will.deacon@arm.com,
arnd@arndb.de, longman@redhat.com, linux-arch@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de,
hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com,
jglauber@marvell.com, steven.sistare@oracle.com,
daniel.m.jordan@oracle.com, dave.dice@oracle.com,
rahul.x.yadav@oracle.com
Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock
Date: Wed, 17 Jul 2019 17:09:13 +0200 [thread overview]
Message-ID: <20190717150913.GY3419@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <FFC2D45A-24B3-40E1-ABBB-1D696E830B23@oracle.com>
On Wed, Jul 17, 2019 at 10:27:30AM -0400, Alex Kogan wrote:
> > On Jul 17, 2019, at 4:39 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > static void cna_splice_tail(struct cna_node *cn, struct cna_node *head, struct cna_node *tail)
> > {
> > struct cna_node *list;
> >
> > /* remove [head,tail] */
> > WRITE_ONCE(cn->mcs.next, tail->mcs.next);
> > tail->mcs.next = NULL;
> >
> > /* stick [head,tail] on the secondary list tail */
> > if (cn->mcs.locked <= 1) {
> > /* create secondary list */
> > head->tail = tail;
> > cn->mcs.locked = head->encoded_tail;
> > } else {
> > /* add to tail */
> > list = (struct cna_node *)decode_tail(cn->mcs.locked);
> > list->tail->next = head;
> > list->tail = tail;
> > }
> > }
> >
> > static struct cna_node *cna_find_next(struct mcs_spinlock *node)
> > {
> > struct cna_node *cni, *cn = (struct cna_node *)node;
> > struct cna_node *head, *tail = NULL;
> >
> > /* find any next lock from 'our' node */
> > for (head = cni = (struct cna_node *)READ_ONCE(cn->mcs.next);
> > cni && cni->node != cn->node;
> > tail = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
> > ;
> >
> > /* when found, splice any skipped locks onto the secondary list */
> > if (cni && tail)
> > cna_splice_tail(cn, head, tail);
> >
> > return cni;
> > }
> >
> > How's that?
>
> This is almost perfect!! :)
>
> The above should work, but I think we should have a specialized fast-path for
> checking the immediate next node in the main queue. This would be the common
> case, once we splice ‘other’ nodes onto the secondary queue. In the above we
> would go through four branches before returning from cna_find_next(). In the
> following we would have just one:
Right, but can you measure a difference? ;-) Anyway, no real objection,
just playing devils advocate here.
> > static struct cna_node *cna_find_next(struct mcs_spinlock *node)
> > {
> > struct cna_node *cn = (struct cna_node *)node;
> struct cna_node *cni = (struct cna_node *)READ_ONCE(cn->mcs.next);
> >
> > struct cna_node *head, *tail = NULL;
> >
> /* fast path */
> if (cni->node == cn->node)
> return cni;
>
> > /* find any next lock from 'our' node */
> > for (head = cn->mcs.next;
head = cni,
you just did that load :-)
> > cni && cni->node != cn->node;
> > tail = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
> > ;
> >
> > /* when found, splice any skipped locks onto the secondary list */
> > if (cni && tail)
> > cna_splice_tail(cn, head, tail);
> >
> > return cni;
> > }
>
>
> Also, any reason you say ‘lock’ instead of ’node’ in the comments?
> I.e., I think "when found, splice any skipped locks onto the secondary list” should be
> "when found, splice any skipped nodes onto the secondary list”.
Due to the confusion between lock waiter node and numa node :-)
next prev parent reply other threads:[~2019-07-17 15:09 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-15 19:25 [PATCH v3 0/5] Add NUMA-awareness to qspinlock Alex Kogan
2019-07-15 19:25 ` [PATCH v3 1/5] locking/qspinlock: Make arch_mcs_spin_unlock_contended more generic Alex Kogan
2019-07-16 10:23 ` Peter Zijlstra
2019-07-15 19:25 ` [PATCH v3 2/5] locking/qspinlock: Refactor the qspinlock slow path Alex Kogan
2019-07-16 10:20 ` Peter Zijlstra
2019-07-16 14:53 ` Alex Kogan
2019-07-16 15:58 ` Peter Zijlstra
2019-07-15 19:25 ` [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Alex Kogan
2019-07-15 21:30 ` Waiman Long
2019-07-16 11:04 ` Peter Zijlstra
2019-07-16 14:26 ` Alex Kogan
2019-07-16 14:44 ` Waiman Long
[not found] ` <aa73b86d-902a-bb6f-d372-8645c8299a6d@redhat.com>
[not found] ` <C1C55A40-FDB1-43B5-B551-F9B8BE776DF8@oracle.com>
2019-07-16 14:50 ` Waiman Long
2019-07-17 17:44 ` Alex Kogan
2019-07-17 17:58 ` Waiman Long
2019-07-16 11:05 ` Peter Zijlstra
2019-07-16 14:30 ` Alex Kogan
2019-07-16 15:50 ` Peter Zijlstra
2019-07-16 17:19 ` Alex Kogan
2019-07-16 18:47 ` Peter Zijlstra
2019-07-17 8:39 ` Peter Zijlstra
2019-07-17 8:59 ` Peter Zijlstra
2019-07-17 14:52 ` Alex Kogan
[not found] ` <FFC2D45A-24B3-40E1-ABBB-1D696E830B23@oracle.com>
2019-07-17 15:09 ` Peter Zijlstra [this message]
2019-07-17 2:16 ` Waiman Long
2019-07-17 7:44 ` Peter Zijlstra
2019-07-17 13:35 ` Waiman Long
2019-07-17 14:42 ` Alex Kogan
2019-07-15 19:25 ` [PATCH v3 4/5] locking/qspinlock: Introduce starvation avoidance into CNA Alex Kogan
2019-07-16 15:59 ` Peter Zijlstra
2019-07-15 19:25 ` [PATCH v3 5/5] locking/qspinlock: Introduce the shuffle reduction optimization " Alex Kogan
2019-07-16 11:47 ` [PATCH v3 0/5] Add NUMA-awareness to qspinlock Nicholas Piggin
[not found] ` <7D29555E-8F72-4EDD-8A87-B1A59C3945A6@oracle.com>
2019-07-16 23:07 ` Nicholas Piggin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190717150913.GY3419@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=alex.kogan@oracle.com \
--cc=arnd@arndb.de \
--cc=bp@alien8.de \
--cc=daniel.m.jordan@oracle.com \
--cc=dave.dice@oracle.com \
--cc=guohanjun@huawei.com \
--cc=hpa@zytor.com \
--cc=jglauber@marvell.com \
--cc=linux-arch@vger.kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@armlinux.org.uk \
--cc=longman@redhat.com \
--cc=mingo@redhat.com \
--cc=rahul.x.yadav@oracle.com \
--cc=steven.sistare@oracle.com \
--cc=tglx@linutronix.de \
--cc=will.deacon@arm.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).