netdev - Re: [PATCH] tools/runqslower: use _

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Date:   Tue, 6 Jul 2021 16:11:19 -0700
From:   Song Liu <song@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Yonghong Song <yhs@...com>, SanjayKumar J <vjsanjay@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] tools/runqslower: use __state instead of state

On Tue, Jul 6, 2021 at 3:05 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Jul 6, 2021 at 11:26 AM Yonghong Song <yhs@...com> wrote:
> >
> >
> >
> > On 7/6/21 10:44 AM, SanjayKumar J wrote:
> > >       task->state is renamed to task->__state in task_struct
> >
> > Could you add a reference to
> >    2f064a59a11f ("sched: Change task_struct::state")
> > which added this change?
> >
> > I think this should go to bpf tree as the change is in linus tree now.
> > Could you annotate the tag as "[PATCH bpf]" ("[PATCH bpf v2]")?
> >
> > Please align comments to the left without margins.
> >
> > >
> > >       Signed-off-by: SanjayKumar J <vjsanjay@...il.com>
> >
> > This Singed-off-by is not needed.
> >
> > You can add my Ack in the next revision:
> > Acked-by: Yonghong Song <yhs@...com>
> >
> > >
> > > Signed-off-by: SanjayKumar J <vjsanjay@...il.com>
> > > ---
> > >   tools/bpf/runqslower/runqslower.bpf.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c
> > > index 645530ca7e98..ab9353f2fd46 100644
> > > --- a/tools/bpf/runqslower/runqslower.bpf.c
> > > +++ b/tools/bpf/runqslower/runqslower.bpf.c
> > > @@ -74,7 +74,7 @@ int handle__sched_switch(u64 *ctx)
> > >       u32 pid;
> > >
> > >       /* ivcsw: treat like an enqueue event and store timestamp */
> > > -     if (prev->state == TASK_RUNNING)
> > > +     if (prev->__state == TASK_RUNNING)
> >
> > Currently, runqslower.bpf.c uses vmlinux.h.
> > I am thinking to use bpf_core_field_exists(), but we need to
> > single out task_struct structure from vmlinux.h
> > with both state and __state fields, we could make it work
> > by *changes* like
> >
> > #define task_struct task_struct_orig
> > #include "vmlinux.h"
> > #undef task_struct
> >
> > struct task_struct {
> >     ... state;
> >     ... __state;
> > ...
> > };
>
>
> no need for such surgery, recommended way is to use ___suffix to
> declare incompatible struct definition:
>
> struct task_struct___old {
>     int state;
> };
>
> Then do casting in BPF code. We don't have to do it in kernel tree's
> runqslower, but we'll definitely have to do that for libbpf-tools'
> runqslower and runqlat.

Question on this topic: state and __state are of different sizes here. IIUC,
bpf_core_types_are_compat() does allow size mismatch. But it may cause
problems in some cases, no? For example, would some combination make
task->state return 32 extra bits from another field and cause confusion?

Thanks,
Song