On Tue, Feb 18, 2020 at 8:34 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 2/18/20 7:56 AM, Daniel Borkmann wrote:
> > On 2/18/20 4:43 PM, Yonghong Song wrote:
> >> On 2/14/20 2:43 PM, Brian Vazquez wrote:
> >>> Grabbing the spinlock for every bucket even if it's empty, was causing
> >>> significant perfomance cost when traversing htab maps that have only a
> >>> few entries. This patch addresses the issue by checking first the
> >>> bucket_cnt, if the bucket has some entries then we go and grab the
> >>> spinlock and proceed with the batching.
> >>>
> >>> Tested with a htab of size 50K and different value of populated entries.
> >>>
> >>> Before:
> >>> Benchmark Time(ns) CPU(ns)
> >>> ---------------------------------------------
> >>> BM_DumpHashMap/1 2759655 2752033
> >>> BM_DumpHashMap/10 2933722 2930825
> >>> BM_DumpHashMap/200 3171680 3170265
> >>> BM_DumpHashMap/500 3639607 3635511
> >>> BM_DumpHashMap/1000 4369008 4364981
> >>> BM_DumpHashMap/5k 11171919 11134028
> >>> BM_DumpHashMap/20k 69150080 69033496
> >>> BM_DumpHashMap/39k 190501036 190226162
> >>>
> >>> After:
> >>> Benchmark Time(ns) CPU(ns)
> >>> ---------------------------------------------
> >>> BM_DumpHashMap/1 202707 200109
> >>> BM_DumpHashMap/10 213441 210569
> >>> BM_DumpHashMap/200 478641 472350
> >>> BM_DumpHashMap/500 980061 967102
> >>> BM_DumpHashMap/1000 1863835 1839575
> >>> BM_DumpHashMap/5k 8961836 8902540
> >>> BM_DumpHashMap/20k 69761497 69322756
> >>> BM_DumpHashMap/39k 187437830 186551111
> >>>
> >>> Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> >>> Cc: Yonghong Song <yhs@xxxxxx>
> >>> Signed-off-by: Brian Vazquez <brianvv@xxxxxxxxxx>
> >>
> >> Acked-by: Yonghong Song <yhs@xxxxxx>
> >
> > I must probably be missing something, but how is this safe? Presume we
> > traverse in the walk with bucket_cnt = 0. Meanwhile a different CPU added
> > entries to this bucket since not locked. Same reader on the other CPU with
> > bucket_cnt = 0 then starts to traverse the second
> > hlist_nulls_for_each_entry_safe() unlocked e.g. deleting entries?
>
> Thanks for pointing this out. Yes, you are correct. If bucket_cnt is 0
> and buck->lock is not held, we should skip the
> hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
> ...
> }
> as another cpu may traverse the bucket in parallel by adding/deleting
> the elements.
Makes sense. Let me fix it in the next version, thanks for reviewing it!
