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 <[email protected]>
> ---
> 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?
>
> @@ -91,7 +95,7 @@ int handle__sched_switch(u64 *ctx)
> bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU,
> &event, sizeof(event));
>
> - bpf_map_delete_elem(&start, &pid);
> + bpf_task_storage_delete(&start, next);
> return 0;
> }
>
>
> On Jan 11, 2021, at 9:49 AM, Yonghong Song <[email protected]> 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 <[email protected]>
>> ---
>> 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?
Thanks,
Song
On 1/11/21 2:54 PM, Song Liu wrote:
>
>
>> On Jan 11, 2021, at 9:49 AM, Yonghong Song <[email protected]> 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 <[email protected]>
>>> ---
>>> 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
>
On Mon, Jan 11, 2021 at 7:24 PM Yonghong Song <[email protected]> wrote:
>
>
>
> On 1/11/21 2:54 PM, Song Liu wrote:
> >
> >
> >> On Jan 11, 2021, at 9:49 AM, Yonghong Song <[email protected]> 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 <[email protected]>
> >>> ---
> >>> 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.
I think Song is right. Given wakeup and sched_switch need to be
matched, depending at which exact time we attach BPF programs, we can
end up missing wakeup, but not missing sched_switch, no? So it's not
an error.
>
> 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
> >
On 1/11/21 11:14 PM, Andrii Nakryiko wrote:
> On Mon, Jan 11, 2021 at 7:24 PM Yonghong Song <[email protected]> wrote:
>>
>>
>>
>> On 1/11/21 2:54 PM, Song Liu wrote:
>>>
>>>
>>>> On Jan 11, 2021, at 9:49 AM, Yonghong Song <[email protected]> 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 <[email protected]>
>>>>> ---
>>>>> 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.
>
> I think Song is right. Given wakeup and sched_switch need to be
> matched, depending at which exact time we attach BPF programs, we can
> end up missing wakeup, but not missing sched_switch, no? So it's not
> an error.
The current approach works fine. What I suggested is to
tighten sched_switch only for target_pid. wakeup (doing queuing) will
be more relaxed than sched_switch to ensure task local storage creation
is always there for target_pid regardless of attachment timing.
I think it should work, but we have to experiment to see actual
results...
>
>>
>> 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
>>>