Messages in this thread Patch in this message | | | Date | Tue, 02 Jul 2013 11:38:55 +0300 | From | Eliezer Tamir <> | Subject | Re: [PATCH net-next 0/2] net: lls cleanup patches |
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On 02/07/2013 00:08, David Miller wrote: > From: Eliezer Tamir <eliezer.tamir@linux.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@linux.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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