On Fri, Dec 09, 2016 at 05:58:34PM +0000, Gleb Smirnoff wrote:
> Author: glebius
> Date: Fri Dec 9 17:58:34 2016
> New Revision: 309745
> URL: https://svnweb.freebsd.org/changeset/base/309745
>
> Log:
> Provide counter_ratecheck(), a MP-friendly substitution to ppsratecheck().
> When rated event happens at a very quick rate, the ppsratecheck() is not
> only racy, but also becomes a performance bottleneck.
>
> Together with: rrs, jtl
>
> Modified:
> head/share/man/man9/counter.9
> head/sys/kern/subr_counter.c
> head/sys/sys/counter.h
>
> Modified: head/share/man/man9/counter.9
> ==============================================================================
> --- head/share/man/man9/counter.9 Fri Dec 9 17:58:07 2016
> (r309744)
> +++ head/share/man/man9/counter.9 Fri Dec 9 17:58:34 2016
> (r309745)
> @@ -25,7 +25,7 @@
> .\"
> .\" $FreeBSD$
> .\"
> -.Dd March 14, 2016
> +.Dd December 8, 2016
> .Dt COUNTER 9
> .Os
> .Sh NAME
> @@ -51,6 +51,8 @@
> .Fn counter_u64_fetch "counter_u64_t c"
> .Ft void
> .Fn counter_u64_zero "counter_u64_t c"
> +.Ft int64_t
> +.Fn counter_ratecheck "struct counter_rate *cr" "int64_t limit"
> .In sys/sysctl.h
> .Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
> .Fn SYSCTL_ADD_COUNTER_U64 ctx parent nbr name access ptr descr
> @@ -128,6 +130,18 @@ value for any moment.
> Clear the counter
> .Fa c
> and set it to zero.
> +.It Fn counter_ratecheck cr limit
> +The function is a multiprocessor-friendly version of
> +.Fn ppsratecheck ,
> +which uses
> +.Nm
> +internally.
> +Returns non-negative value if the rate isn't yet reached during the current
> +second, and a negative value otherwise.
> +If the limit was reached on previous second, but was just reset back to zero,
> +then
> +.Fn counter_ratecheck
> +returns number of events since previous reset.
> .It Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
> Declare a static
> .Xr sysctl
>
> Modified: head/sys/kern/subr_counter.c
> ==============================================================================
> --- head/sys/kern/subr_counter.c Fri Dec 9 17:58:07 2016
> (r309744)
> +++ head/sys/kern/subr_counter.c Fri Dec 9 17:58:34 2016
> (r309745)
> @@ -119,3 +119,57 @@ sysctl_handle_counter_u64_array(SYSCTL_H
>
> return (0);
> }
> +
> +/*
> + * MP-friendly version of ppsratecheck().
> + *
> + * Returns non-negative if we are in the rate, negative otherwise.
> + * 0 - rate limit not reached.
> + * -1 - rate limit reached.
> + * >0 - rate limit was reached before, and was just reset. The return value
> + * is number of events since last reset.
> + */
> +int64_t
> +counter_ratecheck(struct counter_rate *cr, int64_t limit)
> +{
> + int64_t val;
> + int now;
> +
> + val = cr->cr_over;
> + now = ticks;
> +
> + if (now - cr->cr_ticks >= hz) {
> + /*
> + * Time to clear the structure, we are in the next second.
> + * First try unlocked read, and then proceed with atomic.
> + */
> + if ((cr->cr_lock == 0) &&
> + atomic_cmpset_int(&cr->cr_lock, 0, 1)) {
This should be cmpset_acq to avoid reordering of the operations from
locked region outside of it, and to make _rel below to sync-with.
Or call atomic_thread_fence_acq() after cmpset(), then unneeded barrier
is not executed if atomic failed.
> + /*
> + * Check if other thread has just went through the
> + * reset sequence before us.
> + */
> + if (now - cr->cr_ticks >= hz) {
> + val = counter_u64_fetch(cr->cr_rate);
> + counter_u64_zero(cr->cr_rate);
> + cr->cr_over = 0;
> + cr->cr_ticks = now;
> + }
> + atomic_store_rel_int(&cr->cr_lock, 0);
> + } else
> + /*
> + * We failed to lock, in this case other thread may
> + * be running counter_u64_zero(), so it is not safe
> + * to do an update, we skip it.
> + */
> + return (val);
> + }
> +
> + counter_u64_add(cr->cr_rate, 1);
What prevents this thread to fail the check for
now - cr->cr_ticks >= hz
above, then get off the cpu long enough for the next second to tick,
so that another thread starts the cleanup, while this thread performs the
increment ? The current thread update is lost then.
> + if (cr->cr_over != 0)
> + return (-1);
> + if (counter_u64_fetch(cr->cr_rate) > limit)
> + val = cr->cr_over = -1;
> +
> + return (val);
> +}
>
> Modified: head/sys/sys/counter.h
> ==============================================================================
> --- head/sys/sys/counter.h Fri Dec 9 17:58:07 2016 (r309744)
> +++ head/sys/sys/counter.h Fri Dec 9 17:58:34 2016 (r309745)
> @@ -59,5 +59,18 @@ uint64_t counter_u64_fetch(counter_u64_t
> for (int i = 0; i < (n); i++) \
> counter_u64_zero((a)[i]); \
> } while (0)
> +
> +/*
> + * counter(9) based rate checking.
> + */
> +struct counter_rate {
> + counter_u64_t cr_rate; /* Events since last second */
> + volatile int cr_lock; /* Lock to clean the struct */
> + int cr_ticks; /* Ticks on last clean */
> + int cr_over; /* Over limit since cr_ticks? */
> +};
> +
> +int64_t counter_ratecheck(struct counter_rate *, int64_t);
> +
> #endif /* _KERNEL */
> #endif /* ! __SYS_COUNTER_H__ */
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