On Tue, Aug 25, 2020 at 02:55:53PM -0700, Paul E. McKenney wrote:
> On Mon, Aug 24, 2020 at 10:48:42PM -0400, Joel Fernandes (Google) wrote:
> > Track how the segcb list changes before/after acceleration, during
> > queuing and during dequeuing.
> >
> > This has proved useful to discover an optimization to avoid unwanted GP
> > requests when there are no callbacks accelerated. The overhead is minimal as
> > each segment's length is now stored in the respective segment.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> Speaking of tracing, diagnostics, and debugging, a few instances of
> ASSERT_EXCLUSIVE_WRITER() in the previous patch might save a lot
> of debugging. Help KCSAN help us.
Ok, I will look into it, thanks.
> > include/trace/events/rcu.h | 25 +++++++++++++++++++++++++
> > kernel/rcu/rcu_segcblist.c | 27 +++++++++++++++++++++++++++
> > kernel/rcu/rcu_segcblist.h | 7 +++++++
> > kernel/rcu/tree.c | 23 +++++++++++++++++++++++
> > 4 files changed, 82 insertions(+)
> >
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 155b5cb43cfd..7b84df3c95df 100644
> > --- a/include/trace/events/rcu.h
> > +++ b/include/trace/events/rcu.h
> > @@ -505,6 +505,31 @@ TRACE_EVENT_RCU(rcu_callback,
> > __entry->qlen)
> > );
> >
> > +TRACE_EVENT_RCU(rcu_segcb,
> > +
> > + TP_PROTO(const char *ctx, int *cb_count, unsigned long *gp_seq),
> > +
> > + TP_ARGS(ctx, cb_count, gp_seq),
> > +
> > + TP_STRUCT__entry(
> > + __field(const char *, ctx)
> > + __array(int, cb_count, 4)
> > + __array(unsigned long, gp_seq, 4)
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->ctx = ctx;
> > + memcpy(__entry->cb_count, cb_count, 4 * sizeof(int));
> > + memcpy(__entry->gp_seq, gp_seq, 4 * sizeof(unsigned long));
> > + ),
> > +
> > + TP_printk("%s cb_count: (DONE=%d, WAIT=%d, NEXT_READY=%d, NEXT=%d) "
> > + "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx,
> > + __entry->cb_count[0], __entry->cb_count[1], __entry->cb_count[2], __entry->cb_count[3],
> > + __entry->gp_seq[0], __entry->gp_seq[1], __entry->gp_seq[2], __entry->gp_seq[3])
> > +
> > +);
> > +
> > /*
> > * Tracepoint for the registration of a single RCU callback of the special
> > * kvfree() form. The first argument is the RCU type, the second argument
> > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> > index 73a103464ea4..6419dbbaecde 100644
> > --- a/kernel/rcu/rcu_segcblist.c
> > +++ b/kernel/rcu/rcu_segcblist.c
> > @@ -378,6 +378,33 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> > rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
> > }
> >
> > +/*
> > + * Return how many CBs each segment along with their gp_seq values.
> > + *
> > + * This function is O(N) where N is the number of callbacks. Only used from
> > + * tracing code which is usually disabled in production.
> > + */
> > +#ifdef CONFIG_RCU_TRACE
> > +void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> > + int cbcount[RCU_CBLIST_NSEGS],
> > + unsigned long gpseq[RCU_CBLIST_NSEGS])
> > +{
>
> Why not declare the arrays here and invoke the new trace event from
> here also? That would simplify the usage and save a few lines of code.
True! I will do it that way.
thanks,
- Joel
