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Date:   Tue, 29 Mar 2022 19:47:16 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Song Liu <song@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf] tools/runqslower: fix handle__sched_switch for
 updated tp sched_switch

On Tue, Mar 29, 2022 at 5:39 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Mar 29, 2022, at 5:00 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> >
> > On Tue, Mar 29, 2022 at 4:19 PM Song Liu <song@...nel.org> wrote:
> >>
> >> TP_PROTO of sched_switch is updated with a new arg prev_state, which
> >> causes runqslower load failure:
> >>
> >> libbpf: prog 'handle__sched_switch': BPF program load failed: Permission denied
> >> libbpf: prog 'handle__sched_switch': -- BEGIN PROG LOAD LOG --
> >> R1 type=ctx expected=fp
> >> 0: R1=ctx(off=0,imm=0) R10=fp0
> >> ; int handle__sched_switch(u64 *ctx)
> >> 0: (bf) r7 = r1                       ; R1=ctx(off=0,imm=0) R7_w=ctx(off=0,imm=0)
> >> ; struct task_struct *next = (struct task_struct *)ctx[2];
> >> 1: (79) r6 = *(u64 *)(r7 +16)
> >> func 'sched_switch' arg2 has btf_id 186 type STRUCT 'task_struct'
> >> 2: R6_w=ptr_task_struct(off=0,imm=0) R7_w=ctx(off=0,imm=0)
> >> ; struct task_struct *prev = (struct task_struct *)ctx[1];
> >> 2: (79) r2 = *(u64 *)(r7 +8)          ; R2_w=scalar() R7_w=ctx(off=0,imm=0)
> >> 3: (b7) r1 = 0                        ; R1_w=0
> >> ; struct runq_event event = {};
> >> 4: (7b) *(u64 *)(r10 -8) = r1         ; R1_w=P0 R10=fp0 fp-8_w=00000000
> >> 5: (7b) *(u64 *)(r10 -16) = r1        ; R1_w=P0 R10=fp0 fp-16_w=00000000
> >> 6: (7b) *(u64 *)(r10 -24) = r1        ; R1_w=P0 R10=fp0 fp-24_w=00000000
> >> 7: (7b) *(u64 *)(r10 -32) = r1        ; R1_w=P0 R10=fp0 fp-32_w=00000000
> >> ; if (prev->__state == TASK_RUNNING)
> >> 8: (61) r1 = *(u32 *)(r2 +24)
> >> R2 invalid mem access 'scalar'
> >> processed 9 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >> -- END PROG LOAD LOG --
> >> libbpf: failed to load program 'handle__sched_switch'
> >> libbpf: failed to load object 'runqslower_bpf'
> >> libbpf: failed to load BPF skeleton 'runqslower_bpf': -13
> >> failed to load BPF object: -13
> >>
> >> Update runqslower to fix this issue. Also, as we are on this, use BPF_PROG
> >> in runqslower for cleaner code.
> >>
> >> Fixes: fa2c3254d7cf ("sched/tracing: Don't re-read p->state when emitting sched_switch event")
> >> Signed-off-by: Song Liu <song@...nel.org>
> >> ---
> >> tools/bpf/runqslower/runqslower.bpf.c | 19 +++++--------------
> >> 1 file changed, 5 insertions(+), 14 deletions(-)
> >>
> >
> > It would be much less disruptive if that prev_state was added after
> > "next", but oh well...
>
> Maybe we should change that.
>
> +Valentin and Steven, how about we change the order with the attached
> diff (not the original patch in this thread, but the one at the end of
> this email)?
>
> >
> > But anyways, let's handle this in a way that can handle both old
> > kernels and new ones and do the same change in libbpf-tool's
> > runqslower ([0]). Can you please follow up there as well?
>
> Yeah, I will also fix that one.

Thanks!

>
> >
> >
> > We can use BPF CO-RE to detect which order of arguments running kernel
> > has by checking prev_state field existence in struct
> > trace_event_raw_sched_switch. Can you please try that? Use
> > bpf_core_field_exists() for that.
>
> Do you mean something like
>
> if (bpf_core_field_exists(ctx->prev_state))
>     /* use ctx[2] and ctx[3] */
> else
>     /* use ctx[1] and ctx[2] */

yep, that's what I meant, except you don't have ctx->prev_state, you have to do:

if (bpf_core_field_exists(((struct trace_event_raw_sched_switch
*)0)->prev_state))

>
> ? I think we will need BTF for the arguments, which doesn't exist yet.
> Did I miss something?

Probably :) struct trace_event_raw_sched_switch is in vmlinux.h
already for non-raw sched:sched_switch tracepoint. We just use that
type information for feature probing. From BPF verifier's perspective,
ctx[1] or ctx[2] will have proper BTF information (task_struct) during
program validation.

>
> I was thinking about adding something like struct tp_sched_switch_args
> for all the raw tracepoints, but haven't got time to look into how.
>
> Thanks,
> Song
>
> >
> >
> >  [0] https://github.com/iovisor/bcc/blob/master/libbpf-tools/runqslower.bpf.c
> >
> >
> >> diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c
> >> index 9a5c1f008fe6..30e491d8308f 100644
> >> --- a/tools/bpf/runqslower/runqslower.bpf.c
> >> +++ b/tools/bpf/runqslower/runqslower.bpf.c
> >> @@ -2,6 +2,7 @@
> >> // Copyright (c) 2019 Facebook
> >> #include "vmlinux.h"
> >> #include <bpf/bpf_helpers.h>
> >> +#include <bpf/bpf_tracing.h>
> >> #include "runqslower.h"
> >>

[...]

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