Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock

Paul E. McKenney
2020-01-25T00:57:27Z

On Fri, Jan 24, 2020 at 06:39:02PM -0500, Alex Kogan wrote:
> Hi, Paul.
>
> Thanks for running those experiments!
>
> > On Jan 24, 2020, at 5:24 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Tue, Jan 14, 2020 at 10:59:15PM -0500, Alex Kogan wrote:
> >> Minor changes from v8 based on feedback from Longman:
> >> -----------------------------------------------------
> >>
> >> - Add __init to cna_configure_spin_lock_slowpath().
> >>
> >> - Fix the comment for cna_scan_main_queue().
> >>
> >> - Change the type of intra_node_handoff_threshold to unsigned int.
> >>
> >>
> >> Summary
> >> -------
> >>
> >> Lock throughput can be increased by handing a lock to a waiter on the
> >> same NUMA node as the lock holder, provided care is taken to avoid
> >> starvation of waiters on other NUMA nodes. This patch introduces CNA
> >> (compact NUMA-aware lock) as the slow path for qspinlock. It is
> >> enabled through a configuration option (NUMA_AWARE_SPINLOCKS).
> >>
> >> CNA is a NUMA-aware version of the MCS lock. Spinning threads are
> >> organized in two queues, a main queue for threads running on the same
> >> node as the current lock holder, and a secondary queue for threads
> >> running on other nodes. Threads store the ID of the node on which
> >> they are running in their queue nodes. After acquiring the MCS lock and
> >> before acquiring the spinlock, the lock holder scans the main queue
> >> looking for a thread running on the same node (pre-scan). If found (call
> >> it thread T), all threads in the main queue between the current lock
> >> holder and T are moved to the end of the secondary queue. If such T
> >> is not found, we make another scan of the main queue after acquiring
> >> the spinlock when unlocking the MCS lock (post-scan), starting at the
> >> node where pre-scan stopped. If both scans fail to find such T, the
> >> MCS lock is passed to the first thread in the secondary queue. If the
> >> secondary queue is empty, the MCS lock is passed to the next thread in the
> >> main queue. To avoid starvation of threads in the secondary queue, those
> >> threads are moved back to the head of the main queue after a certain
> >> number of intra-node lock hand-offs.
> >>
> >> More details are available at https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=1KUGGZYTHnQ25fgRFppdNvpJfI0rOO_Usdu18RDu_14&s=F12nhHutwnPNt_TQ2ELER0DhtsHlEI9EiW1nDPhm5-Y&e= <https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=1KUGGZYTHnQ25fgRFppdNvpJfI0rOO_Usdu18RDu_14&s=F12nhHutwnPNt_TQ2ELER0DhtsHlEI9EiW1nDPhm5-Y&e=> .
> >>
> >> The series applies on top of v5.5.0-rc6, commit b3a987b026.
> >> Performance numbers are available in previous revisions
> >> of the series.
> >>
> >> Further comments are welcome and appreciated.
> >
> > I ran this on a large system with a version of locktorture that was
> > modified to print out the maximum and minimum per-CPU lock-acquisition
> > counts, and with CPU hotplug disabled. I also modified the LOCK01 and
> > LOCK04 scenarios to use 220 hardware threads.
> >
> > Here is what the test ended up with at the end of a one-hour run:
> >
> > LOCK01 (exclusive):
> > Writes: Total: 1241107333 Max/Min: 9206962/60902 ??? Fail: 0
> >
> > LOCK04 (rwlock):
> > Writes: Total: 232991963 Max/Min: 2631574/74582 ??? Fail: 0
> > Reads : Total: 216935386 Max/Min: 2735939/28665 ??? Fail: 0
> >
> > The "???" strings are printed because the ratio of maximum to minimum exceeds
> > a factor of two.
> Is this what you expect / have seen with the existing qspinlock?
>
> >
> > I also ran 30-minute runs on my laptop, which has 12 hardware threads:
> >
> > LOCK01 (exclusive):
> > Writes: Total: 3992072782 Max/Min: 259368782/97231961 ??? Fail: 0
> >
> > LOCK04 (rwlock):
> > Writes: Total: 131063892 Max/Min: 13136206/5876157 ??? Fail: 0
> > Reads : Total: 144876801 Max/Min: 19999535/4873442 ??? Fail: 0
> I assume the system above is multi-socket, but your laptop is probably not?
>
> If that’s the case, CNA should not be enabled on your laptop (grep
> kernel logs for "Enabling CNA spinlock” to be sure).
>
> >
> > These also exceed the factor-of-two cutoff, but not as dramatically.
> > The readers for the reader-writer lock fared worst, with a 4-to-1 ratio.
> >
> > These tests did run within guest OSes.
> So I really wonder if CNA was enabled here, or whether this is what you get
> with paravirt qspinlock.
>
> > Is that configuration out of
> > scope for this locking algorithm? In addition (as might well also have
> > been the case for the locktorture runs in your paper), these tests run
> > a pair of stress-test tasks for each hardware thread.
> >
> > Is this expected behavior?
> The results do appear skewed a bit too much, but it would be helpful to know
> what qspinlock we are looking at, and how they compare to the existing qspinlock,
> in case it is indeed CNA.

You called it! I will play with QEMU's -numa argument to see if I can get
CNA to run for me. Please accept my apologies for the false alarm.

Thanx, Paul