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Message-ID: <51CCD812.5090408@codyps.com>
Date: Thu, 27 Jun 2013 17:25:54 -0700
From: Cody P Schafer <devel-lists@...yps.com>
To: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
CC: David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org,
Jesse Brandeburg <jesse.brandeburg@...el.com>,
Don Skidmore <donald.c.skidmore@...el.com>,
e1000-devel@...ts.sourceforge.net,
Willem de Bruijn <willemb@...gle.com>,
Eric Dumazet <erdnetdev@...il.com>,
Ben Hutchings <bhutchings@...arflare.com>,
Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
Eilon Greenstien <eilong@...adcom.com>,
Or Gerlitz <or.gerlitz@...il.com>,
Amir Vadai <amirv@...lanox.com>,
Alex Rosenbaum <alexr@...lanox.com>,
Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v4 net-next] net: poll/select low latency socket support
On 06/24/2013 12:28 AM, Eliezer Tamir wrote:
> select/poll busy-poll support.
...
> diff --git a/fs/select.c b/fs/select.c
> index 8c1c96c..79b876e 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -400,6 +402,8 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
> poll_table *wait;
> int retval, i, timed_out = 0;
> unsigned long slack = 0;
> + unsigned int ll_flag = POLL_LL;
> + u64 ll_time = ll_end_time();
>
> rcu_read_lock();
> retval = max_select_fd(n, fds);
> @@ -750,6 +768,8 @@ static int do_poll(unsigned int nfds, struct poll_list *list,
> ktime_t expire, *to = NULL;
> int timed_out = 0, count = 0;
> unsigned long slack = 0;
> + unsigned int ll_flag = POLL_LL;
> + u64 ll_time = ll_end_time();
>
> /* Optimise the no-wait case */
> if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
> diff --git a/include/net/ll_poll.h b/include/net/ll_poll.h
> index fcc7c36..5bf2b3a 100644
> --- a/include/net/ll_poll.h
> +++ b/include/net/ll_poll.h
> @@ -38,17 +39,18 @@ extern unsigned int sysctl_net_ll_poll __read_mostly;
>
> /* we can use sched_clock() because we don't care much about precision
> * we only care that the average is bounded
> + * we don't mind a ~2.5% imprecision so <<10 instead of *1000
> + * sk->sk_ll_usec is a u_int so this can't overflow
> */
> -static inline u64 ll_end_time(struct sock *sk)
> +static inline u64 ll_sk_end_time(struct sock *sk)
> {
> - u64 end_time = ACCESS_ONCE(sk->sk_ll_usec);
> -
> - /* we don't mind a ~2.5% imprecision
> - * sk->sk_ll_usec is a u_int so this can't overflow
> - */
> - end_time = (end_time << 10) + sched_clock();
> + return ((u64)ACCESS_ONCE(sk->sk_ll_usec) << 10) + sched_clock();
> +}
>
> - return end_time;
> +/* in poll/select we use the global sysctl_net_ll_poll value */
> +static inline u64 ll_end_time(void)
> +{
> + return ((u64)ACCESS_ONCE(sysctl_net_ll_poll) << 10) + sched_clock();
> }
>
> static inline bool sk_valid_ll(struct sock *sk)
> @@ -62,10 +64,13 @@ static inline bool can_poll_ll(u64 end_time)
> return !time_after64(sched_clock(), end_time);
> }
>
> +/* when used in sock_poll() nonblock is known at compile time to be true
> + * so the loop and end_time will be optimized out
> + */
> static inline bool sk_poll_ll(struct sock *sk, int nonblock)
> {
> + u64 end_time = nonblock ? 0 : ll_sk_end_time(sk);
> const struct net_device_ops *ops;
> - u64 end_time = ll_end_time(sk);
> struct napi_struct *napi;
> int rc = false;
>
I'm seeing warnings about using smp_processor_id() while preemptable
(log included below) due to this patch. I expect the use of
ll_end_time() -> sched_clock() here is triggering this.
Apologies if this has already been noted.
--
# [ 3.114452] BUG: using smp_processor_id() in preemptible [00000000]
code: sh/62
[ 3.117970] caller is native_sched_clock+0x20/0x80
[ 3.120303] CPU: 0 PID: 62 Comm: sh Not tainted
3.10.0-rc6-dnuma-01032-g2d48d67 #21
[ 3.123710] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
[ 3.128616] 0000000000000000 ffff880002b6baf0 ffffffff813c07d0
ffff880002b6bb08
[ 3.135055] ffffffff811ff835 00000004d076eeed ffff880002b6bb20
ffffffff81009ac0
[ 3.137359] 0000000000000000 ffff880002b6bb30 ffffffff81009b29
ffff880002b6bf40
[ 3.138954] Call Trace:
[ 3.139466] [<ffffffff813c07d0>] dump_stack+0x19/0x1b
[ 3.140559] [<ffffffff811ff835>] debug_smp_processor_id+0xd5/0xf0
[ 3.141831] [<ffffffff81009ac0>] native_sched_clock+0x20/0x80
[ 3.143031] [<ffffffff81009b29>] sched_clock+0x9/0x10
[ 3.144127] [<ffffffff811033a6>] do_sys_poll+0x1f6/0x500
[ 3.145239] [<ffffffff81009b29>] ? sched_clock+0x9/0x10
[ 3.146335] [<ffffffff81009ac0>] ? native_sched_clock+0x20/0x80
[ 3.147557] [<ffffffff8106cf5d>] ? sched_clock_local+0x1d/0x90
[ 3.148816] [<ffffffff81009ac0>] ? native_sched_clock+0x20/0x80
[ 3.150007] [<ffffffff81009b29>] ? sched_clock+0x9/0x10
[ 3.151090] [<ffffffff8106cf5d>] ? sched_clock_local+0x1d/0x90
[ 3.152419] [<ffffffff81009ac0>] ? native_sched_clock+0x20/0x80
[ 3.153638] [<ffffffff81009ac0>] ? native_sched_clock+0x20/0x80
[ 3.154865] [<ffffffff81009b29>] ? sched_clock+0x9/0x10
[ 3.155961] [<ffffffff8106cf5d>] ? sched_clock_local+0x1d/0x90
[ 3.157230] [<ffffffff8106d128>] ? sched_clock_cpu+0xa8/0x100
[ 3.158433] [<ffffffff81101af0>] ? SyS_getdents64+0x110/0x110
[ 3.159628] [<ffffffff81009ac0>] ? native_sched_clock+0x20/0x80
[ 3.160916] [<ffffffff81009b29>] ? sched_clock+0x9/0x10
[ 3.162003] [<ffffffff8106cf5d>] ? sched_clock_local+0x1d/0x90
[ 3.163207] [<ffffffff8106d128>] ? sched_clock_cpu+0xa8/0x100
[ 3.164427] [<ffffffff81084b39>] ? get_lock_stats+0x19/0x60
[ 3.165580] [<ffffffff81084fbe>] ? put_lock_stats.isra.28+0xe/0x40
[ 3.166856] [<ffffffff813c2415>] ? __mutex_unlock_slowpath+0x105/0x1a0
[ 3.168412] [<ffffffff81087c55>] ? trace_hardirqs_on_caller+0x105/0x1d0
[ 3.169944] [<ffffffff81087d2d>] ? trace_hardirqs_on+0xd/0x10
[ 3.171155] [<ffffffff813c24b9>] ? mutex_unlock+0x9/0x10
[ 3.172355] [<ffffffff81251fd3>] ? tty_ioctl+0xa53/0xd40
[ 3.173483] [<ffffffff8108ae28>] ? lock_release_non_nested+0x308/0x350
[ 3.174848] [<ffffffff81089bd6>] ? __lock_acquire+0x3d6/0xb70
[ 3.176087] [<ffffffff81087c55>] ? trace_hardirqs_on_caller+0x105/0x1d0
[ 3.177466] [<ffffffff81101205>] ? do_vfs_ioctl+0x305/0x510
[ 3.178629] [<ffffffff813c6959>] ? sysret_check+0x22/0x5d
[ 3.179764] [<ffffffff81087c55>] ? trace_hardirqs_on_caller+0x105/0x1d0
[ 3.181196] [<ffffffff81103770>] SyS_poll+0x60/0xf0
[ 3.182225] [<ffffffff813c692d>] system_call_fastpath+0x1a/0x1f
--
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