Messages in this thread Patch in this message |  | | Date | Wed, 13 Feb 2013 04:09:44 -0800 | From | tip-bot for Eric Dumazet <> | Subject | [tip:core/locking] x86/smp: Keep spinlock delay values per hashed spinlock address |
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Commit-ID: e3c94ba97ac742a4c03713629da131fef53b2237 Gitweb: http://git.kernel.org/tip/e3c94ba97ac742a4c03713629da131fef53b2237 Author: Eric Dumazet <eric.dumazet@gmail.com> AuthorDate: Wed, 6 Feb 2013 15:06:47 -0500 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Wed, 13 Feb 2013 09:06:30 +0100
x86/smp: Keep spinlock delay values per hashed spinlock address
Eric Dumazet found a regression with the first version of the spinlock backoff code, in a workload where multiple spinlocks were contended, each having a different wait time.
This patch has multiple delay values per cpu, indexed on a hash of the lock address, to avoid that problem.
Eric Dumazet wrote:
I did some tests with your patches with following configuration:
tc qdisc add dev eth0 root htb r2q 1000 default 3 (to force a contention on qdisc lock, even with a multi queue net device)
and 24 concurrent "netperf -t UDP_STREAM -H other_machine -- -m 128"
Machine : 2 Intel(R) Xeon(R) CPU X5660 @ 2.80GHz (24 threads), and a fast NIC (10Gbps)
Resulting in a 13 % regression (676 Mbits -> 595 Mbits)
In this workload we have at least two contended spinlocks, with different delays. (spinlocks are not held for the same duration)
It clearly defeats your assumption of a single per cpu delay being OK : Some cpus are spinning too long while the lock was released.
We might try to use a hash on lock address, and an array of 16 different delays so that different spinlocks have a chance of not sharing the same delay.
With following patch, I get 982 Mbits/s with same bench, so an increase of 45 % instead of a 13 % regression.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: Rik van Riel <riel@redhat.com> Acked-by: Rafael Aquini <aquini@redhat.com> Reviewed-by: Michel Lespinasse <walken@google.com> Cc: lwoodman@redhat.com Cc: knoel@redhat.com Cc: chegu_vinod@hp.com Cc: raghavendra.kt@linux.vnet.ibm.com Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20130206150647.633f7b1c@cuia.bos.redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org> --- arch/x86/kernel/smp.c | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c index c1fce41..8e94469 100644 --- a/arch/x86/kernel/smp.c +++ b/arch/x86/kernel/smp.c @@ -23,6 +23,7 @@ #include <linux/interrupt.h> #include <linux/cpu.h> #include <linux/gfp.h> +#include <linux/hash.h> #include <asm/mtrr.h> #include <asm/tlbflush.h> @@ -116,6 +117,18 @@ static bool smp_no_nmi_ipi = false; #define DELAY_FIXED_1 (1<<DELAY_SHIFT) #define MIN_SPINLOCK_DELAY (1 * DELAY_FIXED_1) #define MAX_SPINLOCK_DELAY (16000 * DELAY_FIXED_1) +#define DELAY_HASH_SHIFT 6 +struct delay_entry { + u32 hash; + u32 delay; +}; +static DEFINE_PER_CPU(struct delay_entry [1 << DELAY_HASH_SHIFT], spinlock_delay) = { + [0 ... (1 << DELAY_HASH_SHIFT) - 1] = { + .hash = 0, + .delay = MIN_SPINLOCK_DELAY, + }, +}; + /* * Wait on a congested ticket spinlock. Many spinlocks are embedded in * data structures; having many CPUs pounce on the cache line with the @@ -134,12 +147,14 @@ static bool smp_no_nmi_ipi = false; * the queue, to slowly increase the delay if we sleep for too short a * time, and to decrease the delay if we slept for too long. */ -DEFINE_PER_CPU(unsigned, spinlock_delay) = { MIN_SPINLOCK_DELAY }; void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc) { __ticket_t head = inc.head, ticket = inc.tail; __ticket_t waiters_ahead; - unsigned delay = __this_cpu_read(spinlock_delay); + u32 hash = hash32_ptr(lock); + u32 slot = hash_32(hash, DELAY_HASH_SHIFT); + struct delay_entry *ent = &__get_cpu_var(spinlock_delay[slot]); + u32 delay = (ent->hash == hash) ? ent->delay : MIN_SPINLOCK_DELAY; unsigned loops; for (;;) { @@ -171,11 +186,12 @@ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc) * to get it back to a good value quickly. */ if (delay >= 2 * DELAY_FIXED_1) - delay -= max(delay/32, DELAY_FIXED_1); + delay -= max_t(u32, delay/32, DELAY_FIXED_1); break; } } - __this_cpu_write(spinlock_delay, delay); + ent->hash = hash; + ent->delay = delay; } /*
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