[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <e890e08e-99d0-9d81-b835-c3a1b4b8bbbf@fb.com>
Date: Mon, 11 Jan 2021 19:24:23 -0800
From: Yonghong Song <yhs@...com>
To: Song Liu <songliubraving@...com>
CC: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"mingo@...hat.com" <mingo@...hat.com>,
"peterz@...radead.org" <peterz@...radead.org>,
"ast@...nel.org" <ast@...nel.org>,
"daniel@...earbox.net" <daniel@...earbox.net>,
"andrii@...nel.org" <andrii@...nel.org>,
"john.fastabend@...il.com" <john.fastabend@...il.com>,
"kpsingh@...omium.org" <kpsingh@...omium.org>,
Kernel Team <Kernel-team@...com>,
"haoluo@...gle.com" <haoluo@...gle.com>
Subject: Re: [PATCH bpf-next 4/4] bpf: runqslower: use task local storage
On 1/11/21 2:54 PM, Song Liu wrote:
>
>
>> On Jan 11, 2021, at 9:49 AM, Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 1/8/21 3:19 PM, Song Liu wrote:
>>> Replace hashtab with task local storage in runqslower. This improves the
>>> performance of these BPF programs. The following table summarizes average
>>> runtime of these programs, in nanoseconds:
>>> task-local hash-prealloc hash-no-prealloc
>>> handle__sched_wakeup 125 340 3124
>>> handle__sched_wakeup_new 2812 1510 2998
>>> handle__sched_switch 151 208 991
>>> Note that, task local storage gives better performance than hashtab for
>>> handle__sched_wakeup and handle__sched_switch. On the other hand, for
>>> handle__sched_wakeup_new, task local storage is slower than hashtab with
>>> prealloc. This is because handle__sched_wakeup_new accesses the data for
>>> the first time, so it has to allocate the data for task local storage.
>>> Once the initial allocation is done, subsequent accesses, as those in
>>> handle__sched_wakeup, are much faster with task local storage. If we
>>> disable hashtab prealloc, task local storage is much faster for all 3
>>> functions.
>>> Signed-off-by: Song Liu <songliubraving@...com>
>>> ---
>>> tools/bpf/runqslower/runqslower.bpf.c | 26 +++++++++++++++-----------
>>> 1 file changed, 15 insertions(+), 11 deletions(-)
>>> diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c
>>> index 1f18a409f0443..c4de4179a0a17 100644
>>> --- a/tools/bpf/runqslower/runqslower.bpf.c
>>> +++ b/tools/bpf/runqslower/runqslower.bpf.c
>>> @@ -11,9 +11,9 @@ const volatile __u64 min_us = 0;
>>> const volatile pid_t targ_pid = 0;
>>> struct {
>>> - __uint(type, BPF_MAP_TYPE_HASH);
>>> - __uint(max_entries, 10240);
>>> - __type(key, u32);
>>> + __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
>>> + __uint(map_flags, BPF_F_NO_PREALLOC);
>>> + __type(key, int);
>>> __type(value, u64);
>>> } start SEC(".maps");
>>> @@ -25,15 +25,19 @@ struct {
>>> /* record enqueue timestamp */
>>> __always_inline
>>> -static int trace_enqueue(u32 tgid, u32 pid)
>>> +static int trace_enqueue(struct task_struct *t)
>>> {
>>> - u64 ts;
>>> + u32 pid = t->pid;
>>> + u64 ts, *ptr;
>>> if (!pid || (targ_pid && targ_pid != pid))
>>> return 0;
>>> ts = bpf_ktime_get_ns();
>>> - bpf_map_update_elem(&start, &pid, &ts, 0);
>>> + ptr = bpf_task_storage_get(&start, t, 0,
>>> + BPF_LOCAL_STORAGE_GET_F_CREATE);
>>> + if (ptr)
>>> + *ptr = ts;
>>> return 0;
>>> }
>>> @@ -43,7 +47,7 @@ int handle__sched_wakeup(u64 *ctx)
>>> /* TP_PROTO(struct task_struct *p) */
>>> struct task_struct *p = (void *)ctx[0];
>>> - return trace_enqueue(p->tgid, p->pid);
>>> + return trace_enqueue(p);
>>> }
>>> SEC("tp_btf/sched_wakeup_new")
>>> @@ -52,7 +56,7 @@ int handle__sched_wakeup_new(u64 *ctx)
>>> /* TP_PROTO(struct task_struct *p) */
>>> struct task_struct *p = (void *)ctx[0];
>>> - return trace_enqueue(p->tgid, p->pid);
>>> + return trace_enqueue(p);
>>> }
>>> SEC("tp_btf/sched_switch")
>>> @@ -70,12 +74,12 @@ int handle__sched_switch(u64 *ctx)
>>> /* ivcsw: treat like an enqueue event and store timestamp */
>>> if (prev->state == TASK_RUNNING)
>>> - trace_enqueue(prev->tgid, prev->pid);
>>> + trace_enqueue(prev);
>>> pid = next->pid;
>>> /* fetch timestamp and calculate delta */
>>> - tsp = bpf_map_lookup_elem(&start, &pid);
>>> + tsp = bpf_task_storage_get(&start, next, 0, 0);
>>> if (!tsp)
>>> return 0; /* missed enqueue */
>>
>> Previously, hash table may overflow so we may have missed enqueue.
>> Here with task local storage, is it possible to add additional pid
>> filtering in the beginning of handle__sched_switch such that
>> missed enqueue here can be treated as an error?
>
> IIUC, hashtab overflow is not the only reason of missed enqueue. If the
> wakeup (which calls trace_enqueue) happens before runqslower starts, we
> may still get missed enqueue in sched_switch, no?
the wakeup won't happen before runqslower starts since runqslower needs
to start to do attachment first and then trace_enqueue() can run.
For the current implementation trace_enqueue() will happen for any non-0
pid before setting test_progs tgid, and will happen for any non-0 and
test_progs tgid if it is set, so this should be okay if we do filtering
in handle__sched_switch. Maybe you can do an experiment to prove whether
my point is correct or not.
>
> Thanks,
> Song
>
Powered by blists - more mailing lists