On Sun, Sep 06, 2020 at 06:06:39PM +0200, Mateusz Nosek wrote: > Hi, > > I performed simple benchmarks using custom kernel module with the code > fragment in question 'copy-pasted' in there in both versions. In case of 1k, > 10k and 100k iterations the average time for kzalloc version was 5.1 and for > kmalloc 3.9, for each iterations number. > The time was measured using 'ktime_get(void)' function and the results given > here are in ktime_t units. > The machine I use has 4 core Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz CPU. > > The performance increase happens, but as you wrote it is probably not really > noticeable.
I don't think that saving a few cylces of memset() in a function that called only on the initialization path in very particular cases is worth risking uninitialized variables when somebody will add a new field to the 'struct mmu_notifier_subscriptions' and will forget to explicitly set it. > I have found 3 other places in kernel code with similar kzalloc related > issues, none of which seems to be 'hot' code. > I leave the decision if this patch and potential others I would send > regarding this issue, are worth applying to the community and maintainers. > > Best regards, > Mateusz Nosek > > On 9/6/2020 4:26 PM, Mike Rapoport wrote: > > Hi, > > > > On Sun, Sep 06, 2020 at 01:43:21PM +0200, mateusznos...@gmail.com wrote: > > > From: Mateusz Nosek <mateusznos...@gmail.com> > > > > > > Most fields in struct pointed by 'subscriptions' are initialized > > > explicitly > > > after the allocation. By changing kzalloc to kmalloc the call to memset > > > is avoided. As the only new code consists of 2 simple memory accesses, > > > the performance is increased. > > > > Is there a measurable performance increase? > > > > The __mmu_notifier_register() is not used that frequently to trade off > > robustness of kzalloc() for slight (if visible at all) performance gain. > > > > > Signed-off-by: Mateusz Nosek <mateusznos...@gmail.com> > > > --- > > > mm/mmu_notifier.c | 4 +++- > > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c > > > index 4fc918163dd3..190e198dc5be 100644 > > > --- a/mm/mmu_notifier.c +++ b/mm/mmu_notifier.c @@ -625,7 +625,7 > > > @@ int __mmu_notifier_register(struct mmu_notifier *subscription, > > > * know that mm->notifier_subscriptions can't change while we * > > > hold the write side of the mmap_lock. */ > > > - subscriptions = kzalloc( > > > + subscriptions = kmalloc( > > > sizeof(struct mmu_notifier_subscriptions), > > > GFP_KERNEL); > > > if (!subscriptions) > > > return -ENOMEM; > > > @@ -636,6 +636,8 @@ int __mmu_notifier_register(struct mmu_notifier > > > *subscription, > > > subscriptions->itree = RB_ROOT_CACHED; > > > init_waitqueue_head(&subscriptions->wq); > > > INIT_HLIST_HEAD(&subscriptions->deferred_list); > > > + subscriptions->active_invalidate_ranges = 0; > > > + subscriptions->has_itree = false; > > > } > > > ret = mm_take_all_locks(mm); > > > -- > > > 2.20.1 > > > > > > > > -- Sincerely yours, Mike.
