Messages in this thread | | | From | (Eric W. Biederman) | Date | Wed, 10 Mar 2021 15:57:31 -0600 | Subject | Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct |
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Thomas Gleixner <tglx@linutronix.de> writes:
> On Thu, Mar 04 2021 at 21:58, Thomas Gleixner wrote: >> On Thu, Mar 04 2021 at 13:04, Eric W. Biederman wrote: >>> Thomas Gleixner <tglx@linutronix.de> writes: >>>> >>>> We could of course do the caching unconditionally for all tasks. >>> >>> Is there any advantage to only doing this for realtime tasks? >> >> It was mostly to avoid tons of cached entries hanging around all over >> the place. So I limited it to the case which the RT users deeply cared >> about. Also related to the accounting question below. >> >>> If not it probably makes sense to do the caching for all tasks. >>> >>> I am wondering if we want to count the cached sigqueue structure to the >>> users rt signal rlimit? >> >> That makes some sense, but that's a user visible change as a single >> signal will up the count for a tasks lifetime while today it is removed >> from accounting again once the signal is delivered. So that needs some >> thought. > > Thought more about it. To make this accounting useful we'd need: > > - a seperate user::sigqueue_cached counter > - a seperate RLIMIT_SIGQUEUE_CACHED > > Then you need to think about the defaults. Any signal heavy application > will want this enabled and obviously automagically. > > Also there is an argument not to have this due to possible pointless > memory consumption. > > But what are we talking about? 80 bytes worth of memory per task in the > worst case. Which is compared to the rest of a task's memory consumption > just noise. > > Looking at some statistics from a devel system there are less than 10 > items cached when the machine is fully idle after boot. During a kernel > compile the cache utilization goes up to ~150 at max (make -j128 and 64 > CPUs). What's interesting is the allocation statistics after boot and > full kernel compile: > > from slab: 23996 > from task cache: 52223 > > A typical pattern there is: > > <ls>-58490 [010] d..2 7765.664198: __sigqueue_alloc: 58488 from slab ffff8881132df460 10 > <ls>-58488 [002] d..1 7765.664294: __sigqueue_free.part.35: cache ffff8881132df460 10 > <ls>-58488 [002] d..2 7765.665146: __sigqueue_alloc: 1149 from cache ffff8881103dc550 10 > bash-1149 [000] d..2 7765.665220: exit_task_sighand: free ffff8881132df460 8 9 > bash-1149 [000] d..1 7765.665662: __sigqueue_free.part.35: cache ffff8881103dc550 9 > > 58488 grabs the sigqueue from bash's task cache and bash sticks it back > in. Lather, rinse and repeat. > > IMO, not bothering with an extra counter and rlimit plus the required > atomic operations is just fine and having this for all tasks > unconditionally looks like a clear win. > > I'll post an updated version of this soonish.
That looks like a good analysis.
I see that there is a sigqueue_cachep. As I recall there are per cpu caches and all kinds of other good stuff when using kmem_cache_alloc.
Are those goodies falling down?
I am just a little unclear on why a slab allocation is sufficiently problematic that we want to avoid it.
Eric
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