Re: [RFC/PATCH] perf lock contention: Add -J/--inject-delay option

[Namhyung Kim <namhyung@kernel.org>]

Hi Arnaldo,

On Thu, May 08, 2025 at 11:45:58AM -0300, Arnaldo Carvalho de Melo wrote:
> On Mon, Feb 24, 2025 at 11:59:29PM -0800, Namhyung Kim wrote:
> > This is to slow down lock acquistion (on contention locks) deliberately.
> > A possible use case is to estimate impact on application performance by
> > optimization of kernel locking behavior.  By delaying the lock it can
> > simulate the worse condition as a control group, and then compare with
> > the current behavior as a optimized condition.
> 
> So this looks useful, I guess we can proceed and merge it?

That'd be great. :)

Thanks,
Namhyung

>  
> > The syntax is 'time@function' and the time can have unit suffix like
> > "us" and "ms".  For example, I ran a simple test like below.
> > 
> >   $ sudo perf lock con -abl -L tasklist_lock -- \
> >     sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
> >    contended   total wait     max wait     avg wait            address   symbol
> > 
> >           92      1.18 ms    199.54 us     12.79 us   ffffffff8a806080   tasklist_lock (rwlock)
> > 
> > The contention count was 92 and the average wait time was around 10 us.
> > But if I add 100 usec of delay to the tasklist_lock,
> > 
> >   $ sudo perf lock con -abl -L tasklist_lock -J 100us@tasklist_lock -- \
> >     sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
> >    contended   total wait     max wait     avg wait            address   symbol
> > 
> >          190     15.67 ms    230.10 us     82.46 us   ffffffff8a806080   tasklist_lock (rwlock)
> > 
> > The contention count increased and the average wait time was up closed
> > to 100 usec.  If I increase the delay even more,
> > 
> >   $ sudo perf lock con -abl -L tasklist_lock -J 1ms@tasklist_lock -- \
> >     sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
> >    contended   total wait     max wait     avg wait            address   symbol
> > 
> >         1002      2.80 s       3.01 ms      2.80 ms   ffffffff8a806080   tasklist_lock (rwlock)
> > 
> > Now every sleep process had contention and the wait time was more than 1
> > msec.  This is on my 4 CPU laptop so I guess one CPU has the lock while
> > other 3 are waiting for it mostly.
> > 
> > For simplicity, it only supports global locks for now.
> > 
> > Suggested-by: Stephane Eranian <eranian@google.com>
> > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > ---
> >  tools/perf/Documentation/perf-lock.txt        | 11 +++
> >  tools/perf/builtin-lock.c                     | 74 +++++++++++++++++++
> >  tools/perf/util/bpf_lock_contention.c         | 28 +++++++
> >  .../perf/util/bpf_skel/lock_contention.bpf.c  | 43 +++++++++++
> >  tools/perf/util/lock-contention.h             |  8 ++
> >  5 files changed, 164 insertions(+)
> > 
> > diff --git a/tools/perf/Documentation/perf-lock.txt b/tools/perf/Documentation/perf-lock.txt
> > index d3793054f7d35626..151fc837587b216e 100644
> > --- a/tools/perf/Documentation/perf-lock.txt
> > +++ b/tools/perf/Documentation/perf-lock.txt
> > @@ -215,6 +215,17 @@ CONTENTION OPTIONS
> >  --cgroup-filter=<value>::
> >  	Show lock contention only in the given cgroups (comma separated list).
> >  
> > +-J::
> > +--inject-delay=<time@function>::
> > +	Add delays to the given lock.  It's added to the contention-end part so
> > +	that the (new) owner of the lock will be delayed.  But by slowing down
> > +	the owner, the waiters will also be delayed as well.  This is working
> > +	only with -b/--use-bpf.
> > +
> > +	The 'time' is specified in nsec but it can have a unit suffix.  Available
> > +	units are "ms" and "us".  Note that it will busy-wait after it gets the
> > +	lock.  Please use it at your own risk.
> > +
> >  
> >  SEE ALSO
> >  --------
> > diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> > index 5d405cd8e696d21b..3ef452d5d9f5679d 100644
> > --- a/tools/perf/builtin-lock.c
> > +++ b/tools/perf/builtin-lock.c
> > @@ -62,6 +62,8 @@ static const char *output_name = NULL;
> >  static FILE *lock_output;
> >  
> >  static struct lock_filter filters;
> > +static struct lock_delay *delays;
> > +static int nr_delays;
> >  
> >  static enum lock_aggr_mode aggr_mode = LOCK_AGGR_ADDR;
> >  
> > @@ -1971,6 +1973,8 @@ static int __cmd_contention(int argc, const char **argv)
> >  		.max_stack = max_stack_depth,
> >  		.stack_skip = stack_skip,
> >  		.filters = &filters,
> > +		.delays = delays,
> > +		.nr_delays = nr_delays,
> >  		.save_callstack = needs_callstack(),
> >  		.owner = show_lock_owner,
> >  		.cgroups = RB_ROOT,
> > @@ -2474,6 +2478,74 @@ static int parse_cgroup_filter(const struct option *opt __maybe_unused, const ch
> >  	return ret;
> >  }
> >  
> > +static bool add_lock_delay(char *spec)
> > +{
> > +	char *at, *pos;
> > +	struct lock_delay *tmp;
> > +	unsigned long duration;
> > +
> > +	at = strchr(spec, '@');
> > +	if (at == NULL) {
> > +		pr_err("lock delay should have '@' sign: %s\n", spec);
> > +		return false;
> > +	}
> > +	if (at == spec) {
> > +		pr_err("lock delay should have time before '@': %s\n", spec);
> > +		return false;
> > +	}
> > +
> > +	*at = '\0';
> > +	duration = strtoul(spec, &pos, 0);
> > +	if (!strcmp(pos, "ns"))
> > +		duration *= 1;
> > +	else if (!strcmp(pos, "us"))
> > +		duration *= 1000;
> > +	else if (!strcmp(pos, "ms"))
> > +		duration *= 1000 * 1000;
> > +	else if (*pos) {
> > +		pr_err("invalid delay time: %s@%s\n", spec, at + 1);
> > +		return false;
> > +	}
> > +
> > +	tmp = realloc(delays, (nr_delays + 1) * sizeof(*delays));
> > +	if (tmp == NULL) {
> > +		pr_err("Memory allocation failure\n");
> > +		return false;
> > +	}
> > +	delays = tmp;
> > +
> > +	delays[nr_delays].sym = strdup(at + 1);
> > +	if (delays[nr_delays].sym == NULL) {
> > +		pr_err("Memory allocation failure\n");
> > +		return false;
> > +	}
> > +	delays[nr_delays].time = duration;
> > +
> > +	nr_delays++;
> > +	return true;
> > +}
> > +
> > +static int parse_lock_delay(const struct option *opt __maybe_unused, const char *str,
> > +			    int unset __maybe_unused)
> > +{
> > +	char *s, *tmp, *tok;
> > +	int ret = 0;
> > +
> > +	s = strdup(str);
> > +	if (s == NULL)
> > +		return -1;
> > +
> > +	for (tok = strtok_r(s, ", ", &tmp); tok; tok = strtok_r(NULL, ", ", &tmp)) {
> > +		if (!add_lock_delay(tok)) {
> > +			ret = -1;
> > +			break;
> > +		}
> > +	}
> > +
> > +	free(s);
> > +	return ret;
> > +}
> > +
> >  int cmd_lock(int argc, const char **argv)
> >  {
> >  	const struct option lock_options[] = {
> > @@ -2550,6 +2622,8 @@ int cmd_lock(int argc, const char **argv)
> >  	OPT_BOOLEAN(0, "lock-cgroup", &show_lock_cgroups, "show lock stats by cgroup"),
> >  	OPT_CALLBACK('G', "cgroup-filter", NULL, "CGROUPS",
> >  		     "Filter specific cgroups", parse_cgroup_filter),
> > +	OPT_CALLBACK('J', "inject-delay", NULL, "TIME@FUNC",
> > +		     "Inject delays to specific locks", parse_lock_delay),
> >  	OPT_PARENT(lock_options)
> >  	};
> >  
> > diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> > index fc8666222399c995..99b64f303e761cbc 100644
> > --- a/tools/perf/util/bpf_lock_contention.c
> > +++ b/tools/perf/util/bpf_lock_contention.c
> > @@ -183,6 +183,27 @@ int lock_contention_prepare(struct lock_contention *con)
> >  		skel->rodata->has_addr = 1;
> >  	}
> >  
> > +	/* resolve lock name in delays */
> > +	if (con->nr_delays) {
> > +		struct symbol *sym;
> > +		struct map *kmap;
> > +
> > +		for (i = 0; i < con->nr_delays; i++) {
> > +			sym = machine__find_kernel_symbol_by_name(con->machine,
> > +								  con->delays[i].sym,
> > +								  &kmap);
> > +			if (sym == NULL) {
> > +				pr_warning("ignore unknown symbol: %s\n",
> > +					   con->delays[i].sym);
> > +				continue;
> > +			}
> > +
> > +			con->delays[i].addr = map__unmap_ip(kmap, sym->start);
> > +		}
> > +		skel->rodata->lock_delay = 1;
> > +		bpf_map__set_max_entries(skel->maps.lock_delays, con->nr_delays);
> > +	}
> > +
> >  	bpf_map__set_max_entries(skel->maps.cpu_filter, ncpus);
> >  	bpf_map__set_max_entries(skel->maps.task_filter, ntasks);
> >  	bpf_map__set_max_entries(skel->maps.type_filter, ntypes);
> > @@ -272,6 +293,13 @@ int lock_contention_prepare(struct lock_contention *con)
> >  			bpf_map_update_elem(fd, &con->filters->cgrps[i], &val, BPF_ANY);
> >  	}
> >  
> > +	if (con->nr_delays) {
> > +		fd = bpf_map__fd(skel->maps.lock_delays);
> > +
> > +		for (i = 0; i < con->nr_delays; i++)
> > +			bpf_map_update_elem(fd, &con->delays[i].addr, &con->delays[i].time, BPF_ANY);
> > +	}
> > +
> >  	if (con->aggr_mode == LOCK_AGGR_CGROUP)
> >  		read_all_cgroups(&con->cgroups);
> >  
> > diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > index 6533ea9b044c71d1..0ac9ae2f1711a129 100644
> > --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > @@ -11,6 +11,9 @@
> >  /* for collect_lock_syms().  4096 was rejected by the verifier */
> >  #define MAX_CPUS  1024
> >  
> > +/* for do_lock_delay().  Arbitrarily set to 1 million. */
> > +#define MAX_LOOP  (1U << 20)
> > +
> >  /* lock contention flags from include/trace/events/lock.h */
> >  #define LCB_F_SPIN	(1U << 0)
> >  #define LCB_F_READ	(1U << 1)
> > @@ -114,6 +117,13 @@ struct {
> >  	__uint(max_entries, 1);
> >  } slab_caches SEC(".maps");
> >  
> > +struct {
> > +	__uint(type, BPF_MAP_TYPE_HASH);
> > +	__uint(key_size, sizeof(__u64));
> > +	__uint(value_size, sizeof(__u64));
> > +	__uint(max_entries, 1);
> > +} lock_delays SEC(".maps");
> > +
> >  struct rw_semaphore___old {
> >  	struct task_struct *owner;
> >  } __attribute__((preserve_access_index));
> > @@ -143,6 +153,7 @@ const volatile int needs_callstack;
> >  const volatile int stack_skip;
> >  const volatile int lock_owner;
> >  const volatile int use_cgroup_v2;
> > +const volatile int lock_delay;
> >  
> >  /* determine the key of lock stat */
> >  const volatile int aggr_mode;
> > @@ -348,6 +359,35 @@ static inline __u32 check_lock_type(__u64 lock, __u32 flags)
> >  	return 0;
> >  }
> >  
> > +static inline long delay_callback(__u64 idx, void *arg)
> > +{
> > +	__u64 target = *(__u64 *)arg;
> > +
> > +	if (target <= bpf_ktime_get_ns())
> > +		return 1;
> > +
> > +	/* just to kill time */
> > +	(void)bpf_get_prandom_u32();
> > +
> > +	return 0;
> > +}
> > +
> > +static inline void do_lock_delay(__u64 duration)
> > +{
> > +	__u64 target = bpf_ktime_get_ns() + duration;
> > +
> > +	bpf_loop(MAX_LOOP, delay_callback, &target, /*flags=*/0);
> > +}
> > +
> > +static inline void check_lock_delay(__u64 lock)
> > +{
> > +	__u64 *delay;
> > +
> > +	delay = bpf_map_lookup_elem(&lock_delays, &lock);
> > +	if (delay)
> > +		do_lock_delay(*delay);
> > +}
> > +
> >  static inline struct tstamp_data *get_tstamp_elem(__u32 flags)
> >  {
> >  	__u32 pid;
> > @@ -566,6 +606,9 @@ int contention_end(u64 *ctx)
> >  		data->min_time = duration;
> >  
> >  out:
> > +	if (lock_delay)
> > +		check_lock_delay(pelem->lock);
> > +
> >  	pelem->lock = 0;
> >  	if (need_delete)
> >  		bpf_map_delete_elem(&tstamp, &pid);
> > diff --git a/tools/perf/util/lock-contention.h b/tools/perf/util/lock-contention.h
> > index a09f7fe877df8184..12f6cb789ada1bc7 100644
> > --- a/tools/perf/util/lock-contention.h
> > +++ b/tools/perf/util/lock-contention.h
> > @@ -18,6 +18,12 @@ struct lock_filter {
> >  	char			**slabs;
> >  };
> >  
> > +struct lock_delay {
> > +	char			*sym;
> > +	unsigned long		addr;
> > +	unsigned long		time;
> > +};
> > +
> >  struct lock_stat {
> >  	struct hlist_node	hash_entry;
> >  	struct rb_node		rb;		/* used for sorting */
> > @@ -140,6 +146,7 @@ struct lock_contention {
> >  	struct machine *machine;
> >  	struct hlist_head *result;
> >  	struct lock_filter *filters;
> > +	struct lock_delay *delays;
> >  	struct lock_contention_fails fails;
> >  	struct rb_root cgroups;
> >  	unsigned long map_nr_entries;
> > @@ -148,6 +155,7 @@ struct lock_contention {
> >  	int aggr_mode;
> >  	int owner;
> >  	int nr_filtered;
> > +	int nr_delays;
> >  	bool save_callstack;
> >  };
> >  
> > -- 
> > 2.48.1.658.g4767266eb4-goog