Re: [PATCH 4/5] x86,smp: keep spinlock delay values per hashedspinlock address

[Rafael Aquini]

On Tue, Jan 08, 2013 at 05:31:19PM -0500, Rik van Riel wrote:
> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---

Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>


>  arch/x86/kernel/smp.c |   22 +++++++++++++++++++---
>  1 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 05f828b..1877890 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>
> @@ -134,12 +135,26 @@ 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_PER_CPU(unsigned, spinlock_delay) = { MIN_SPINLOCK_DELAY };
> +#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,
> +	},
> +};
> +
>  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 = 1;
>  
>  	for (;;) {
> @@ -175,7 +190,8 @@ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
>  			break;
>  		}
>  	}
> -	__this_cpu_write(spinlock_delay, delay);
> +	ent->hash = hash;
> +	ent->delay = delay;
>  }
>  
>  /*
> 
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