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Message-ID: <ZyqD_sM8U7d1TOqS@google.com>
Date: Tue, 5 Nov 2024 12:45:50 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Adrian Hunter <adrian.hunter@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
linux-perf-users@...r.kernel.org, Song Liu <song@...nel.org>,
bpf@...r.kernel.org, Stephane Eranian <eranian@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>, Kees Cook <kees@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH 3/4] perf lock contention: Resolve slab object name using
BPF
Hi Ian,
On Tue, Nov 05, 2024 at 09:41:05AM -0800, Ian Rogers wrote:
> On Tue, Nov 5, 2024 at 9:26 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > The bpf_get_kmem_cache() kfunc can return an address of the slab cache
> > (kmem_cache). As it has the name of the slab cache from the iterator,
> > we can use it to symbolize some dynamic kernel locks in a slab.
> >
> > Before:
> > root@...tme-ng:/home/namhyung/project/linux# tools/perf/perf lock con -abl sleep 1
> > contended total wait max wait avg wait address symbol
> >
> > 2 3.34 us 2.87 us 1.67 us ffff9d7800ad9600 (mutex)
> > 2 2.16 us 1.93 us 1.08 us ffff9d7804b992d8 (mutex)
> > 4 1.37 us 517 ns 343 ns ffff9d78036e6e00 (mutex)
> > 1 1.27 us 1.27 us 1.27 us ffff9d7804b99378 (mutex)
> > 2 845 ns 599 ns 422 ns ffffffff9e1c3620 delayed_uprobe_lock (mutex)
> > 1 845 ns 845 ns 845 ns ffffffff9da0b280 jiffies_lock (spinlock)
> > 2 377 ns 259 ns 188 ns ffffffff9e1cf840 pcpu_alloc_mutex (mutex)
> > 1 305 ns 305 ns 305 ns ffffffff9e1b4cf8 tracepoint_srcu_srcu_usage (mutex)
> > 1 295 ns 295 ns 295 ns ffffffff9e1c0940 pack_mutex (mutex)
> > 1 232 ns 232 ns 232 ns ffff9d7804b7d8d8 (mutex)
> > 1 180 ns 180 ns 180 ns ffffffff9e1b4c28 tracepoint_srcu_srcu_usage (mutex)
> > 1 165 ns 165 ns 165 ns ffffffff9da8b3a0 text_mutex (mutex)
> >
> > After:
> > root@...tme-ng:/home/namhyung/project/linux# tools/perf/perf lock con -abl sleep 1
> > contended total wait max wait avg wait address symbol
> >
> > 2 1.95 us 1.77 us 975 ns ffff9d5e852d3498 &task_struct (mutex)
> > 1 1.18 us 1.18 us 1.18 us ffff9d5e852d3538 &task_struct (mutex)
> > 4 1.12 us 354 ns 279 ns ffff9d5e841ca800 &kmalloc-cg-512 (mutex)
> > 2 859 ns 617 ns 429 ns ffffffffa41c3620 delayed_uprobe_lock (mutex)
> > 3 691 ns 388 ns 230 ns ffffffffa41c0940 pack_mutex (mutex)
> > 3 421 ns 164 ns 140 ns ffffffffa3a8b3a0 text_mutex (mutex)
> > 1 409 ns 409 ns 409 ns ffffffffa41b4cf8 tracepoint_srcu_srcu_usage (mutex)
> > 2 362 ns 239 ns 181 ns ffffffffa41cf840 pcpu_alloc_mutex (mutex)
> > 1 220 ns 220 ns 220 ns ffff9d5e82b534d8 &signal_cache (mutex)
> > 1 215 ns 215 ns 215 ns ffffffffa41b4c28 tracepoint_srcu_srcu_usage (mutex)
> >
> > Note that the name starts with '&' sign for slab objects to inform they
> > are dynamic locks. It won't give the accurate lock or type names but
> > it's still useful. We may add type info to the slab cache later to get
> > the exact name of the lock in the type later.
>
> Many variables may reference a lock through a pointer, should the name
> not be associated with the lock or from decoding the task_struct?
I'm not sure I understood you correctly. But this only covers when the
lock variable is inside a slab object so that the address falls into the
slab pages.
> The '&' looks redundant as the addresses are clearly different.
Probably. But sometimes users may want clear separation without looking
at the address values. Also we want to use it in the filters and that
would need some form of indication for the slab locks.
> How are >1 lock/mutex in the same struct handled?
It cannot distinguish them for now. It'll be possible once we can have
type info (BTF) for slab objects and a helper to tell the offset inside
the object from the given address. With that, we could have something
like &task_struct.futex_exit_mutex or &signal_struct.cred_guard_mutex.
Thanks,
Namhyung
>
> Thanks,
> Ian
>
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > tools/perf/util/bpf_lock_contention.c | 52 +++++++++++++++++++
> > .../perf/util/bpf_skel/lock_contention.bpf.c | 21 +++++++-
> > 2 files changed, 71 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> > index a2efd40897bad316..50c3039c647d4d77 100644
> > --- a/tools/perf/util/bpf_lock_contention.c
> > +++ b/tools/perf/util/bpf_lock_contention.c
> > @@ -2,6 +2,7 @@
> > #include "util/cgroup.h"
> > #include "util/debug.h"
> > #include "util/evlist.h"
> > +#include "util/hashmap.h"
> > #include "util/machine.h"
> > #include "util/map.h"
> > #include "util/symbol.h"
> > @@ -20,12 +21,25 @@
> >
> > static struct lock_contention_bpf *skel;
> > static bool has_slab_iter;
> > +static struct hashmap slab_hash;
> > +
> > +static size_t slab_cache_hash(long key, void *ctx __maybe_unused)
> > +{
> > + return key;
> > +}
> > +
> > +static bool slab_cache_equal(long key1, long key2, void *ctx __maybe_unused)
> > +{
> > + return key1 == key2;
> > +}
> >
> > static void check_slab_cache_iter(struct lock_contention *con)
> > {
> > struct btf *btf = btf__load_vmlinux_btf();
> > s32 ret;
> >
> > + hashmap__init(&slab_hash, slab_cache_hash, slab_cache_equal, /*ctx=*/NULL);
> > +
> > ret = libbpf_get_error(btf);
> > if (ret) {
> > pr_debug("BTF loading failed: %d\n", ret);
> > @@ -50,6 +64,7 @@ static void run_slab_cache_iter(void)
> > {
> > int fd;
> > char buf[256];
> > + long key, *prev_key;
> >
> > if (!has_slab_iter)
> > return;
> > @@ -65,6 +80,34 @@ static void run_slab_cache_iter(void)
> > continue;
> >
> > close(fd);
> > +
> > + /* Read the slab cache map and build a hash with IDs */
> > + fd = bpf_map__fd(skel->maps.slab_caches);
> > + prev_key = NULL;
> > + while (!bpf_map_get_next_key(fd, prev_key, &key)) {
> > + struct slab_cache_data *data;
> > +
> > + data = malloc(sizeof(*data));
> > + if (data == NULL)
> > + break;
> > +
> > + if (bpf_map_lookup_elem(fd, &key, data) < 0)
> > + break;
> > +
> > + hashmap__add(&slab_hash, data->id, data);
> > + prev_key = &key;
> > + }
> > +}
> > +
> > +static void exit_slab_cache_iter(void)
> > +{
> > + struct hashmap_entry *cur;
> > + unsigned bkt;
> > +
> > + hashmap__for_each_entry(&slab_hash, cur, bkt)
> > + free(cur->pvalue);
> > +
> > + hashmap__clear(&slab_hash);
> > }
> >
> > int lock_contention_prepare(struct lock_contention *con)
> > @@ -398,6 +441,7 @@ static const char *lock_contention_get_name(struct lock_contention *con,
> >
> > if (con->aggr_mode == LOCK_AGGR_ADDR) {
> > int lock_fd = bpf_map__fd(skel->maps.lock_syms);
> > + struct slab_cache_data *slab_data;
> >
> > /* per-process locks set upper bits of the flags */
> > if (flags & LCD_F_MMAP_LOCK)
> > @@ -416,6 +460,12 @@ static const char *lock_contention_get_name(struct lock_contention *con,
> > return "rq_lock";
> > }
> >
> > + /* look slab_hash for dynamic locks in a slab object */
> > + if (hashmap__find(&slab_hash, flags & LCB_F_SLAB_ID_MASK, &slab_data)) {
> > + snprintf(name_buf, sizeof(name_buf), "&%s", slab_data->name);
> > + return name_buf;
> > + }
> > +
> > return "";
> > }
> >
> > @@ -590,5 +640,7 @@ int lock_contention_finish(struct lock_contention *con)
> > cgroup__put(cgrp);
> > }
> >
> > + exit_slab_cache_iter();
> > +
> > return 0;
> > }
> > diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > index fd24ccb00faec0ba..b5bc37955560a58e 100644
> > --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > @@ -123,6 +123,8 @@ struct mm_struct___new {
> > struct rw_semaphore mmap_lock;
> > } __attribute__((preserve_access_index));
> >
> > +extern struct kmem_cache *bpf_get_kmem_cache(u64 addr) __ksym __weak;
> > +
> > /* control flags */
> > const volatile int has_cpu;
> > const volatile int has_task;
> > @@ -496,8 +498,23 @@ int contention_end(u64 *ctx)
> > };
> > int err;
> >
> > - if (aggr_mode == LOCK_AGGR_ADDR)
> > - first.flags |= check_lock_type(pelem->lock, pelem->flags);
> > + if (aggr_mode == LOCK_AGGR_ADDR) {
> > + first.flags |= check_lock_type(pelem->lock,
> > + pelem->flags & LCB_F_TYPE_MASK);
> > +
> > + /* Check if it's from a slab object */
> > + if (bpf_get_kmem_cache) {
> > + struct kmem_cache *s;
> > + struct slab_cache_data *d;
> > +
> > + s = bpf_get_kmem_cache(pelem->lock);
> > + if (s != NULL) {
> > + d = bpf_map_lookup_elem(&slab_caches, &s);
> > + if (d != NULL)
> > + first.flags |= d->id;
> > + }
> > + }
> > + }
> >
> > err = bpf_map_update_elem(&lock_stat, &key, &first, BPF_NOEXIST);
> > if (err < 0) {
> > --
> > 2.47.0.199.ga7371fff76-goog
> >
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