Re: [PATCH v9 4/7] rcu/trace: Add tracing for how segcb list changes

Joel Fernandes
2020-11-04T15:06:02Z

On Wed, Nov 04, 2020 at 07:10:17PM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/3/2020 7:56 PM, 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.
> >
> > Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> > Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> > include/trace/events/rcu.h | 25 +++++++++++++++++++++++++
> > kernel/rcu/rcu_segcblist.c | 34 ++++++++++++++++++++++++++++++++++
> > kernel/rcu/rcu_segcblist.h | 5 +++++
> > kernel/rcu/tree.c | 9 +++++++++
> > 4 files changed, 73 insertions(+)
> >
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 155b5cb43cfd..5f8f2ee1a936 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_stats,
> > +
> > + TP_PROTO(const char *ctx, int *cb_count, unsigned long *gp_seq),
>
> I think we need to use long[] instead of int[] for cb_count everywhere in
> this patch?

More than 4 billion callbacks on a single cblist sounds like a bug though so
int should work. Plus I prefer to keep the tracepoint size small (on 64-bit
systems, long is 64 bits, int is 32 bits).

Thanks for all the reviews on this and other patches!

- Joel

>
>
> Thanks
> Neeraj
>
> > +
> > + TP_ARGS(ctx, cb_count, gp_seq),
> > +
> > + TP_STRUCT__entry(
> > + __field(const char *, ctx)
> > + __array(int, cb_count, RCU_CBLIST_NSEGS)
> > + __array(unsigned long, gp_seq, RCU_CBLIST_NSEGS)
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->ctx = ctx;
> > + memcpy(__entry->cb_count, cb_count, RCU_CBLIST_NSEGS * sizeof(int));
> > + memcpy(__entry->gp_seq, gp_seq, RCU_CBLIST_NSEGS * 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 357c19bbcb00..2a03949d0b82 100644
> > --- a/kernel/rcu/rcu_segcblist.c
> > +++ b/kernel/rcu/rcu_segcblist.c
> > @@ -14,6 +14,7 @@
> > #include <linux/types.h>
> > #include "rcu_segcblist.h"
> > +#include "rcu.h"
> > /* Initialize simple callback list. */
> > void rcu_cblist_init(struct rcu_cblist *rclp)
> > @@ -328,6 +329,39 @@ 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 segments. Only used from
> > + * tracing code which is usually disabled in production.
> > + */
> > +#ifdef CONFIG_RCU_TRACE
> > +static void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> > + int cbcount[RCU_CBLIST_NSEGS],
> > + unsigned long gpseq[RCU_CBLIST_NSEGS])
> > +{
> > + int i;
> > +
> > + for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
> > + cbcount[i] = rcu_segcblist_get_seglen(rsclp, i);
> > + gpseq[i] = rsclp->gp_seq[i];
> > + }
> > +}
> > +
> > +void __trace_rcu_segcb_stats(struct rcu_segcblist *rsclp, const char *context)
> > +{
> > + int cbs[RCU_CBLIST_NSEGS];
> > + unsigned long gps[RCU_CBLIST_NSEGS];
> > +
> > + if (!trace_rcu_segcb_stats_enabled())
> > + return;
> > +
> > + rcu_segcblist_countseq(rsclp, cbs, gps);
> > +
> > + trace_rcu_segcb_stats(context, cbs, gps);
> > +}
> > +#endif
> > +
> > /*
> > * Extract only those callbacks still pending (not yet ready to be
> > * invoked) from the specified rcu_segcblist structure and place them in
> > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> > index cd35c9faaf51..7750734fa116 100644
> > --- a/kernel/rcu/rcu_segcblist.h
> > +++ b/kernel/rcu/rcu_segcblist.h
> > @@ -103,3 +103,8 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq);
> > bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq);
> > void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
> > struct rcu_segcblist *src_rsclp);
> > +#ifdef CONFIG_RCU_TRACE
> > +void __trace_rcu_segcb_stats(struct rcu_segcblist *rsclp, const char *context);
> > +#else
> > +#define __trace_rcu_segcb_stats(...)
> > +#endif
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 24c00020ab83..f6c6653b3ec2 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1497,6 +1497,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> > if (!rcu_segcblist_pend_cbs(&rdp->cblist))
> > return false;
> > + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPreAcc"));
> > +
> > /*
> > * Callbacks are often registered with incomplete grace-period
> > * information. Something about the fact that getting exact
> > @@ -1517,6 +1519,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> > else
> > trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));
> > + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPostAcc"));
> > +
> > return ret;
> > }
> > @@ -2466,11 +2470,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
> > rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
> > if (offloaded)
> > rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
> > +
> > + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbDequeued"));
> > rcu_nocb_unlock_irqrestore(rdp, flags);
> > /* Invoke callbacks. */
> > tick_dep_set_task(current, TICK_DEP_BIT_RCU);
> > rhp = rcu_cblist_dequeue(&rcl);
> > +
> > for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> > rcu_callback_t f;
> > @@ -2983,6 +2990,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> > trace_rcu_callback(rcu_state.name, head,
> > rcu_segcblist_n_cbs(&rdp->cblist));
> > + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
> > +
> > /* Go handle any RCU core processing required. */
> > if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> > __call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
> >
>
> --
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