Re: [RFC PATCH v6 4/5] sched, tracing: add to report task state in symbolic chars
From: Ze Gao
Date: Thu Aug 03 2023 - 06:55:51 EST
On Thu, Aug 3, 2023 at 5:29 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu, 3 Aug 2023 04:33:51 -0400
> Ze Gao <zegao2021@xxxxxxxxx> wrote:
>
> > Internal representations of task state are likely to be changed
> > or ordered, and reporting them to userspace without exporting
> > them as part of API is basically wrong, which can easily break
> > a userspace observability tool as kernel evolves. For example,
> > perf suffers from this and still reports wrong states as of this
> > writing.
> >
> > OTOH, some masqueraded states like TASK_REPORT_IDLE and
> > TASK_REPORT_MAX are also reported inadvertently, which confuses
> > things even more and most userspace tools do not even take them
> > into consideration.
> >
> > So add a new variable in company with the old raw value to
> > report task state in symbolic chars, which are self-explaining
> > and no further translation is needed. Of course this does not
> > break any userspace tool.
> >
> > Note for PREEMPT_ACTIVE, we introduce 'p' to report it and use
> > the old conventions for the rest.
>
> The above is actually good.
>
>
> >
> > Signed-off-by: Ze Gao <zegao@xxxxxxxxxxx>
> > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > Acked-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> > include/trace/events/sched.h | 44 ++++++++++++++++++++++--------------
> > 1 file changed, 27 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> > index 43492daefa6f..ae5b486cc969 100644
> > --- a/include/trace/events/sched.h
> > +++ b/include/trace/events/sched.h
> > @@ -6,6 +6,7 @@
> > #define _TRACE_SCHED_H
> >
> > #include <linux/kthread.h>
> > +#include <linux/sched.h>
> > #include <linux/sched/numa_balancing.h>
> > #include <linux/tracepoint.h>
> > #include <linux/binfmts.h>
> > @@ -214,6 +215,27 @@ static inline short __trace_sched_switch_state(bool preempt,
> >
> > return state ? (1 << (state - 1)) : state;
> > }
> > +
> > +static inline char __trace_sched_switch_state_char(bool preempt,
> > + unsigned int prev_state,
> > + struct task_struct *p)
> > +{
> > + long state;
> > +
> > +#ifdef CONFIG_SCHED_DEBUG
> > + WARN_ON_ONCE(p != current);
> > +#endif /* CONFIG_SCHED_DEBUG */
> > +
> > + /*
> > + * For PREEMPT_ACTIVE, we introduce 'p' to report it and use the old
> > + * conventions for the rest.
> > + */
> > + if (preempt)
> > + return 'p';
> > +
> > + state = __task_state_index(prev_state, p->exit_state);
> > + return task_index_to_char(state);
> > +}
> > #endif /* CREATE_TRACE_POINTS */
> >
> > /*
> > @@ -236,6 +258,7 @@ TRACE_EVENT(sched_switch,
> > __array( char, prev_comm, TASK_COMM_LEN )
> > __array( char, next_comm, TASK_COMM_LEN )
> > __field( short, prev_state )
> > + __field( char, prev_state_char )
> > ),
> >
> > TP_fast_assign(
> > @@ -246,26 +269,13 @@ TRACE_EVENT(sched_switch,
> > memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
> > memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
> > __entry->prev_state = __trace_sched_switch_state(preempt, prev_state, prev);
> > + __entry->prev_state_char = __trace_sched_switch_state_char(preempt, prev_state, prev);
> > /* XXX SCHED_DEADLINE */
> > ),
> >
> > - TP_printk("prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%s%s ==> next_comm=%s next_pid=%d next_prio=%d",
> > - __entry->prev_comm, __entry->prev_pid, __entry->prev_prio,
> > -
> > - (__entry->prev_state & (TASK_REPORT_MAX - 1)) ?
> > - __print_flags(__entry->prev_state & (TASK_REPORT_MAX - 1), "|",
> > - { TASK_INTERRUPTIBLE, "S" },
> > - { TASK_UNINTERRUPTIBLE, "D" },
> > - { __TASK_STOPPED, "T" },
> > - { __TASK_TRACED, "t" },
> > - { EXIT_DEAD, "X" },
> > - { EXIT_ZOMBIE, "Z" },
> > - { TASK_PARKED, "P" },
> > - { TASK_DEAD, "I" }) :
> > - "R",
>
> I just realized, I have user space code that looks at this. As in the format file we have:
>
> print fmt: "prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%s%s ==> next_comm=%s next_pid=%d next_prio=%d", REC->prev_comm, REC->prev_pid, REC->prev_prio, (REC->prev_state & ((((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) - 1)) ? __print_flags(REC->prev_state & ((((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) - 1), "|", { 0x00000001, "S" }, { 0x00000002, "D" }, { 0x00000004, "T" }, { 0x00000008, "t" }, { 0x00000010, "X" }, { 0x00000020, "Z" }, { 0x00000040, "P" }, { 0x00000080, "I" }) : "R", REC->prev_state & (((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) ? "+" : "", REC->next_comm, REC->next_pid, REC->next_prio
>
> And I have used this in applications to find out what values "S" and "D"
> are.
>
> So, we need to keep that still. But we can add the prev_state_char to the
> output too.
>
> "prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%s%s[%c] ==> next_comm=%s next_pid=%d next_prio=%d"
Good point!
But I see Peter has strong opinions over this change badly. So I'm not
sure if it's worth the effort to push this anymore :/ And apparently We
failed to convince each other over this problem.
How about we only keep all the fixing patches and throw away this
'prev_state_char' thing?
Again, I'm not meant to upset anyone here. But Tons of thanks for your
sage reviews on this.
Thoughts?
Thanks,
Ze