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Date: Tue, 02 Jul 2013 11:38:55 +0300
From: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To: David Miller <davem@...emloft.net>
CC: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
willemb@...gle.com, erdnetdev@...il.com, andi@...stfloor.org,
hpa@...or.com, devel-lists@...yps.com, eliezer@...ir.org.il
Subject: Re: [PATCH net-next 0/2] net: lls cleanup patches
On 02/07/2013 00:08, David Miller wrote:
> From: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
> Date: Fri, 28 Jun 2013 15:59:18 +0300
>
>> Here are two cleanup patches.
>>
>> 1. fix warning from debug_smp_processor_id().
>> - reported by Cody P Schafer.
>>
> Applied, but like Ben said perhaps you want to remember the last cpu you
> got the sched_clock() measurement from and abort the ll poll if it changes
> on you instead of using a comparison between two cpus.
>
> But then again, since preemption is enabled, the cpu could change
> back and forth during the sched_clock() call, so you wouldn't be able
> to reliably detect this anyways.
>
> In the grand scheme of things all of this probably doesn't matter at
> all.
The only thing that really worries me, is the possibility of time
on the new cpu to be completely random, then we could be back in the
range where time_after() will be false again and end up polling for
another year.
A simple way to limit the damage would be to use time_in_range()
instead of time_after(), then if we have a completely random time we
would be out of the range and fail safely.
would something like this be an acceptable solution?
---
[PATCH net-next] net: convert lls to use time_in_range()
Time in range will fail safely if we move to a different cpu with an
extremely large clock skew.
Add time_in_range64() and convert lls to use it.
Signed-of-by: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
---
fs/select.c | 10 ++++++----
include/linux/jiffies.h | 4 ++++
include/net/ll_poll.h | 24 +++++++++++++++---------
3 files changed, 25 insertions(+), 13 deletions(-)
diff --git a/fs/select.c b/fs/select.c
index 3654075..f28a585 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -403,7 +403,8 @@ int do_select(int n, fd_set_bits *fds, struct
timespec *end_time)
int retval, i, timed_out = 0;
unsigned long slack = 0;
unsigned int ll_flag = ll_get_flag();
- u64 ll_time = ll_end_time();
+ u64 ll_start = ll_start_time(ll_flag);
+ u64 ll_time = ll_run_time();
rcu_read_lock();
retval = max_select_fd(n, fds);
@@ -498,7 +499,7 @@ int do_select(int n, fd_set_bits *fds, struct
timespec *end_time)
}
/* only if on, have sockets with POLL_LL and not out of time */
- if (ll_flag && can_ll && can_poll_ll(ll_time))
+ if (ll_flag && can_ll && can_poll_ll(ll_start, ll_time))
continue;
/*
@@ -770,7 +771,8 @@ static int do_poll(unsigned int nfds, struct
poll_list *list,
int timed_out = 0, count = 0;
unsigned long slack = 0;
unsigned int ll_flag = ll_get_flag();
- u64 ll_time = ll_end_time();
+ u64 ll_start = ll_start_time(ll_flag);
+ u64 ll_time = ll_run_time();
/* Optimise the no-wait case */
if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
@@ -819,7 +821,7 @@ static int do_poll(unsigned int nfds, struct
poll_list *list,
break;
/* only if on, have sockets with POLL_LL and not out of time */
- if (ll_flag && can_ll && can_poll_ll(ll_time))
+ if (ll_flag && can_ll && can_poll_ll(ll_start, ll_time))
continue;
/*
diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 8fb8edf..37b7354 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -139,6 +139,10 @@ static inline u64 get_jiffies_64(void)
((__s64)(a) - (__s64)(b) >= 0))
#define time_before_eq64(a,b) time_after_eq64(b,a)
+#define time_in_range64(a,b,c) \
+ (time_after_eq64(a,b) && \
+ time_before_eq64(a,c))
+
/*
* These four macros compare jiffies and 'a' for convenience.
*/
diff --git a/include/net/ll_poll.h b/include/net/ll_poll.h
index 6c06f7c..61c2daf 100644
--- a/include/net/ll_poll.h
+++ b/include/net/ll_poll.h
@@ -67,19 +67,23 @@ static inline u64 ll_sched_clock(void)
/* 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_sk_end_time(struct sock *sk)
+static inline u64 ll_sk_run_time(struct sock *sk)
{
- return ((u64)ACCESS_ONCE(sk->sk_ll_usec) << 10) + ll_sched_clock();
+ return (u64)ACCESS_ONCE(sk->sk_ll_usec) << 10;
}
/* in poll/select we use the global sysctl_net_ll_poll value
* only call sched_clock() if enabled
*/
-static inline u64 ll_end_time(void)
+static inline u64 ll_run_time(void)
{
- u64 end_time = ACCESS_ONCE(sysctl_net_ll_poll);
+ return (u64)ACCESS_ONCE(sysctl_net_ll_poll) << 10;
+}
- return end_time ? (end_time << 10) + ll_sched_clock() : 0;
+/* if flag is not set we don't need to know the time */
+static inline u64 ll_start_time(unsigned int flag)
+{
+ return flag ? ll_sched_clock() : 0;
}
static inline bool sk_valid_ll(struct sock *sk)
@@ -88,9 +92,10 @@ static inline bool sk_valid_ll(struct sock *sk)
!need_resched() && !signal_pending(current);
}
-static inline bool can_poll_ll(u64 end_time)
+static inline bool can_poll_ll(u64 start_time, u64 run_time)
{
- return !time_after64(ll_sched_clock(), end_time);
+ return time_in_range64(ll_sched_clock(), start_time,
+ start_time + run_time);
}
/* when used in sock_poll() nonblock is known at compile time to be true
@@ -98,7 +103,8 @@ static inline bool can_poll_ll(u64 end_time)
*/
static inline bool sk_poll_ll(struct sock *sk, int nonblock)
{
- u64 end_time = nonblock ? 0 : ll_sk_end_time(sk);
+ u64 start_time = ll_start_time(!nonblock);
+ u64 run_time = ll_sk_run_time(sk);
const struct net_device_ops *ops;
struct napi_struct *napi;
int rc = false;
@@ -129,7 +135,7 @@ static inline bool sk_poll_ll(struct sock *sk, int
nonblock)
LINUX_MIB_LOWLATENCYRXPACKETS, rc);
} while (!nonblock && skb_queue_empty(&sk->sk_receive_queue) &&
- can_poll_ll(end_time));
+ can_poll_ll(start_time, run_time));
rc = !skb_queue_empty(&sk->sk_receive_queue);
out:
--
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