From: Peter Zijlstra <peterz@infradead.org>
To: Alex Kogan <alex.kogan@oracle.com>
Cc: linux-arch@vger.kernel.org, guohanjun@huawei.com, arnd@arndb.de,
dave.dice@oracle.com, jglauber@marvell.com, x86@kernel.org,
will.deacon@arm.com, linux@armlinux.org.uk,
steven.sistare@oracle.com, linux-kernel@vger.kernel.org,
rahul.x.yadav@oracle.com, mingo@redhat.com, bp@alien8.de,
hpa@zytor.com, longman@redhat.com, tglx@linutronix.de,
daniel.m.jordan@oracle.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 4/5] locking/qspinlock: Introduce starvation avoidance into CNA
Date: Tue, 16 Jul 2019 17:59:00 +0200 [thread overview]
Message-ID: <20190716155900.GS3419@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190715192536.104548-5-alex.kogan@oracle.com>
On Mon, Jul 15, 2019 at 03:25:35PM -0400, Alex Kogan wrote:
> @@ -36,6 +37,33 @@ struct cna_node {
>
> #define CNA_NODE(ptr) ((struct cna_node *)(ptr))
>
> +/* Per-CPU pseudo-random number seed */
> +static DEFINE_PER_CPU(u32, seed);
> +
> +/*
> + * Controls the probability for intra-node lock hand-off. It can be
> + * tuned and depend, e.g., on the number of CPUs per node. For now,
> + * choose a value that provides reasonable long-term fairness without
> + * sacrificing performance compared to a version that does not have any
> + * fairness guarantees.
> + */
> +#define INTRA_NODE_HANDOFF_PROB_ARG 0x10000
> +
> +/*
> + * Return false with probability 1 / @range.
> + * @range must be a power of 2.
> + */
> +static bool probably(unsigned int range)
> +{
> + u32 s;
> +
> + s = this_cpu_read(seed);
> + s = next_pseudo_random32(s);
> + this_cpu_write(seed, s);
> +
> + return s & (range - 1);
This is fragile, better to take a number of bits as argument.
> +}
> +
> static void cna_init_node(struct mcs_spinlock *node)
> {
> struct cna_node *cn = CNA_NODE(node);
> @@ -140,7 +168,13 @@ static inline void cna_pass_mcs_lock(struct mcs_spinlock *node,
> u64 *var = &next->locked;
> u64 val = 1;
>
> - succ = find_successor(node);
> + /*
> + * Try to pass the lock to a thread running on the same node.
> + * For long-term fairness, search for such a thread with high
> + * probability rather than always.
> + */
> + if (probably(INTRA_NODE_HANDOFF_PROB_ARG))
> + succ = find_successor(node);
>
> if (succ) {
> var = &succ->mcs.locked;
And this is where that tertiary condition comes from.. I think.
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next prev parent reply other threads:[~2019-07-16 15:59 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
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 [this message]
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
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