On 7/29/21 3:49 PM, Sebastian Andrzej Siewior wrote: > now that I'm slowly catching up… > > On 2021-07-02 22:25:05 [+0200], Vlastimil Babka wrote: >> > - perf_5.10 stat -r 10 hackbench -g200 -s 4096 -l500 >> > Old: >> > | 464.967,20 msec task-clock # 27,220 CPUs utilized ( +- 0,16% ) >> > New: >> > | 422.865,71 msec task-clock # 4,782 CPUs utilized ( +- 0,34% ) >> >> The series shouldn't significantly change the memory allocator >> interaction, though. >> Seems there's less cycles, but more time elapsed, thus more sleeping - >> is it locks becoming mutexes on RT? > > yes, most likely since the !RT parts are mostly unchanged. > >> My second guess - list_lock remains spinlock with my series, thus RT >> mutex, but the current RT tree converts it to raw_spinlock. I'd hope >> leaving that one as non-raw spinlock would still be much better for RT >> goals, even if hackbench (which is AFAIK very slab intensive) throughput >> regresses - hopefully not that much. > > Yes, the list_lock seems to be the case. I picked your > slub-local-lock-v3r0 and changed the list_lock (+slab_lock()) to use > raw_spinlock_t and disable interrupts and CPUs utilisation went to > ~23CPUs (plus a bunch of warnings which probably made it a little slower > again). I forgot to point that out in the cover letter, but with v3 this change to raw_spinlock_t is AFAICS no longer possible (at least with CONFIG_SLUB_CPU_PARTIAL) because in put_cpu_partial() we now take the local_lock and it can be called from get_partial_node() which takes the list_lock. > The difference between a sleeping lock (spinlock_t) and a mutex is > that we attempt not to preempt a task that acquired a spinlock_t even if > it is running for some time and the scheduler would preempt it (like it > would do if the task had a mutex acquired. These are the "lazy preempt" > bits in the RT patch). > > By making the list_lock a raw_spinlock_t a lot of IRQ-flags dancing > needs to be done as the page-allocator must be entered with enabled > interrupts. Hm but SLUB should never call the page allocator from under list_lock in my series? > And then there is the possibility that you may need to free > some memory even if you allocate memory which requires some extra steps > on RT due to the IRQ-off part. All this vanishes by keeping list_lock a > spinlock_t. > The kernel-build test on /dev/shm remained unchanged so that is good. > Unless there is a real-world use-case, that gets worse, I don't mind > keeping the spinlock_t here. I haven't seen tglx complaining so far. Good. IIRC hackbench is very close to being a slab microbenchmark, so regressions there are expected, but should not translate to notable real world regressions. > Sebastian >
