Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update elem

[Martin KaFai Lau <kafai@fb.com>]

On Tue, Jan 12, 2016 at 07:05:47PM +0800, Ming Lei wrote:
> On Tue, Jan 12, 2016 at 1:49 PM, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> > On Tue, Jan 12, 2016 at 01:00:00PM +0800, Ming Lei wrote:
> >> Hi Alexei,
> >>
> >> Thanks for your review.
> >>
> >> On Tue, Jan 12, 2016 at 3:02 AM, Alexei Starovoitov
> >> <alexei.starovoitov@gmail.com> wrote:
> >> > On Mon, Jan 11, 2016 at 11:56:57PM +0800, Ming Lei wrote:
> >> >> Prepare for supporting percpu map in the following patch.
> >> >>
> >> >> Now userspace can lookup/update mapped value in one specific
> >> >> CPU in case of percpu map.
> >> >>
> >> >> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> >> > ...
> >> >> @@ -265,7 +272,10 @@ static int map_lookup_elem(union bpf_attr *attr)
> >> >>               goto free_key;
> >> >>
> >> >>       rcu_read_lock();
> >> >> -     ptr = map->ops->map_lookup_elem(map, key);
> >> >> +     if (!percpu)
> >> >> +             ptr = map->ops->map_lookup_elem(map, key);
> >> >> +     else
> >> >> +             ptr = map->ops->map_lookup_elem_percpu(map, key, attr->cpu);
> >> >
> >> > I think this approach is less potent than Martin's for several reasons:
> >> > - bpf program shouldn't be supplying bpf_smp_processor_id(), since
> >> >   it's error prone and a bit slower than doing it explicitly as in:
> >> >   https://urldefense.proofpoint.com/v2/url?u=http-3A__patchwork.ozlabs.org_patch_564482_&d=CwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=VQnoQ7LvghIj0gVEaiQSUw&m=kb6DfquDoMLBv0hgOO76O9SMvdCnhwnEwhgON8868I8&s=QtJkMfQDB55jn_aA_umJ8jiJRQlQhW5UxYO5YdxuGNI&e=
> >> >   although Martin's patch also needs to use this_cpu_ptr() instead
> >> >   of per_cpu_ptr(.., smp_processor_id());
> >>
> >> For PERCPU map, smp_processor_id() is definitely required, and
> >> Martin's patch need that too, please see htab_percpu_map_lookup_elem()
> >> in his patch.
> >
> > hmm. it's definitely _not_ required. right?
> > bpf programs shouldn't be accessing other per-cpu regions
> > only their own. That's what this_cpu_ptr is for.
> > I don't see a case where accessing other cpu per-cpu element
> > wouldn't be a bug in the program.
> >
> >> > - two new bpf helpers are not necessary in Martin's approach.
> >> >   regular map_lookup_elem() will work for both per-cpu maps.
> >>
> >> For percpu ARRAY, they are not necessary, but it is flexiable to
> >> provide them since we should allow prog to retrieve the perpcu
> >> value, also it is easier to implement the system call with the two
> >> helpers.
> >>
> >> For percpu HASH, they are required since eBPF prog need to support
> >> deleting element, so we have provide these helpers for prog to retrieve
> >> percpu value before deleting the elem.
> >
> > bpf programs cannot have loops, so there is no valid case to access
> > other cpu element, since program cannot aggregate all-cpu values.
> > Therefore the programs can only update/lookup this_cpu element and
> > delete such element across all cpus.
>
> Looks I missed the point of looping constraint, then basically delete element
> helper doesn't make sense in percpu hash.
>
> >
> >> > - such map_lookup_elem_percpu() from syscall is not accurate.
> >> >   Martin's approach via smp_call_function_single() returns precise value,
> >>
> >> I don't understand why Martin's approach is precise and my patch isn't,
> >> could you explain it a bit?
> >
> > because simple mempcy() called from syscall will race with lookup/increment
> > done to this_cpu element on another cpu. To avoid this race the smp_call
> > is needed, so that memcpy() happens on the cpu that updated the element,
> > so smp_call's memcpy and bpf program won't be touch that cpu value
> > at the same time and user space will read the correct element values.
> > If program updates them a lot, the value that user space reads will become
> > stale very quickly, but it will be valid. That's especially important
> > when program have multiple counters inside single element value.
>
> But smp_call is often very slow because of IPI, so the value acculated
> finally becomes stale easily even though the value from the requested cpu
> is 'precise' at the exact time, especially when there are lots of CPUs, so I
> think using smp_call is really a bad idea. And smp_call is worse than
> iterating from CPUs simply.
The userspace usually only aggregates value across all cpu every X seconds.
I hardly consider some number of micro-seconds old data is stale.