Re: [PATCH v2 bpf-next 4/4] bpf: runqslower: use task local storage
From: Andrii Nakryiko <hidden>
Date: 2021-01-27 21:45:32
Also in:
bpf, lkml
On Tue, Jan 26, 2021 at 1:11 AM Song Liu [off-list ref] 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 <redacted>
---Acked-by: Andrii Nakryiko <andrii@kernel.org>
quoted hunk
tools/bpf/runqslower/runqslower.bpf.c | 31 ++++++++++++++++----------- 1 file changed, 19 insertions(+), 12 deletions(-)diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c index 1f18a409f0443..a597a23d79939 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 +25,18 @@ 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 *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 = bpf_ktime_get_ns();
nit: if (!ptr) return 0; is more in line with general handling of error/unusual conditions. Keeps main logic sequential (especially if we need to add some extra steps later.
quoted hunk
return 0; }@@ -43,7 +46,7 @@ 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")
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