Jack Steiner wrote on Wednesday, November 22, 2006 7:55 AM
> Improve the scalability of the fpswa code that rate-limits
> logging of messages.
>
> There are 2 distinctly different problems in this code.
>
> 1) If prctl is used to disable logging, last_time is never
> updated. The result is that fpu_swa_count is zeroed out on
> EVERY fp fault. This causes a very very hot cache line.
> The fix reduces the wallclock time of a 1024p FP exception test
> from 28734 sec to 19 sec!!!
>
> 2) On VERY large systems, excessive messages are logged because
> multiple cpus can each reset or increment fpu_swa_count at
> about the same time. The result is that hundreds of messages
> are logged each second. The fixes reduces the logging rate
> to ~1 per second.
>
>
> +static DEFINE_PER_CPU(struct fpu_swa_msg, cpulast);
> +DECLARE_PER_CPU(struct fpu_swa_msg, cpulast);
> +static struct fpu_swa_msg last __cacheline_aligned;
>
> [...]
>
> + if (!(current->thread.flags & IA64_THREAD_FPEMU_NOPRINT)) {
> + unsigned long count, current_jiffies = jiffies;
> + struct fpu_swa_msg *cp = &__get_cpu_var(cpulast);
Now that you fixed prctl problem mentioned in (1) above, do you really
have to go such elaborated per-cpu method of updating last_time and
fpu_swa_count? fpu_swa_count should really be declared as atomic_t and
use atomic_inc() for updates. And I would think that should rate limit
properly at 5 per 5 sec.
- Ken
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