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Message-ID: <CAEf4BzbSvMa2+hdTifMKTsNiOL6X=P7eor4LpPKfHM=Y9-71fw@mail.gmail.com>
Date: Fri, 29 Mar 2024 10:33:57 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Jonthan Haslam <jonathan.haslam@...il.com>, linux-trace-kernel@...r.kernel.org,
andrii@...nel.org, bpf@...r.kernel.org, rostedt@...dmis.org,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uprobes: reduce contention on uprobes_tree access
On Wed, Mar 27, 2024 at 5:45 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Mar 27, 2024 at 5:18 PM Masami Hiramatsu <mhiramat@...nelorg> wrote:
> >
> > On Wed, 27 Mar 2024 17:06:01 +0000
> > Jonthan Haslam <jonathan.haslam@...il.com> wrote:
> >
> > > > > Masami,
> > > > >
> > > > > Given the discussion around per-cpu rw semaphore and need for
> > > > > (internal) batched attachment API for uprobes, do you think you can
> > > > > apply this patch as is for now? We can then gain initial improvements
> > > > > in scalability that are also easy to backport, and Jonathan will work
> > > > > on a more complete solution based on per-cpu RW semaphore, as
> > > > > suggested by Ingo.
> > > >
> > > > Yeah, it is interesting to use per-cpu rw semaphore on uprobe.
> > > > I would like to wait for the next version.
> > >
> > > My initial tests show a nice improvement on the over RW spinlocks but
> > > significant regression in acquiring a write lock. I've got a few days
> > > vacation over Easter but I'll aim to get some more formalised results out
> > > to the thread toward the end of next week.
> >
> > As far as the write lock is only on the cold path, I think you can choose
> > per-cpu RW semaphore. Since it does not do busy wait, the total system
> > performance impact will be small.
>
> No, Masami, unfortunately it's not as simple. In BPF we have BPF
> multi-uprobe, which can be used to attach to thousands of user
> functions. It currently creates one uprobe at a time, as we don't
> really have a batched API. If each such uprobe registration will now
> take a (relatively) long time, when multiplied by number of attach-to
> user functions, it will be a horrible regression in terms of
> attachment/detachment performance.
>
> So when we switch to per-CPU rw semaphore, we'll need to provide an
> internal batch uprobe attach/detach API to make sure that attaching to
> multiple uprobes is still fast.
>
> Which is why I was asking to land this patch as is, as it relieves the
> scalability pains in production and is easy to backport to old
> kernels. And then we can work on batched APIs and switch to per-CPU rw
> semaphore.
>
> So I hope you can reconsider and accept improvements in this patch,
> while Jonathan will keep working on even better final solution.
> Thanks!
>
> > I look forward to your formalized results :)
> >
BTW, as part of BPF selftests, we have a multi-attach test for uprobes
and USDTs, reporting attach/detach timings:
$ sudo ./test_progs -v -t uprobe_multi_test/bench
bpf_testmod.ko is already unloaded.
Loading bpf_testmod.ko...
Successfully loaded bpf_testmod.ko.
test_bench_attach_uprobe:PASS:uprobe_multi_bench__open_and_load 0 nsec
test_bench_attach_uprobe:PASS:uprobe_multi_bench__attach 0 nsec
test_bench_attach_uprobe:PASS:uprobes_count 0 nsec
test_bench_attach_uprobe: attached in 0.120s
test_bench_attach_uprobe: detached in 0.092s
#400/5 uprobe_multi_test/bench_uprobe:OK
test_bench_attach_usdt:PASS:uprobe_multi__open 0 nsec
test_bench_attach_usdt:PASS:bpf_program__attach_usdt 0 nsec
test_bench_attach_usdt:PASS:usdt_count 0 nsec
test_bench_attach_usdt: attached in 0.124s
test_bench_attach_usdt: detached in 0.064s
#400/6 uprobe_multi_test/bench_usdt:OK
#400 uprobe_multi_test:OK
Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
Successfully unloaded bpf_testmod.ko.
So it should be easy for Jonathan to validate his changes with this.
> > Thank you,
> >
> > >
> > > Jon.
> > >
> > > >
> > > > Thank you,
> > > >
> > > > >
> > > > > >
> > > > > > BTW, how did you measure the overhead? I think spinlock overhead
> > > > > > will depend on how much lock contention happens.
> > > > > >
> > > > > > Thank you,
> > > > > >
> > > > > > >
> > > > > > > [0] https://docs.kernel.org/locking/spinlocks.html
> > > > > > >
> > > > > > > Signed-off-by: Jonathan Haslam <jonathan.haslam@...il.com>
> > > > > > > ---
> > > > > > > kernel/events/uprobes.c | 22 +++++++++++-----------
> > > > > > > 1 file changed, 11 insertions(+), 11 deletions(-)
> > > > > > >
> > > > > > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > > > > > > index 929e98c62965..42bf9b6e8bc0 100644
> > > > > > > --- a/kernel/events/uprobes.c
> > > > > > > +++ b/kernel/events/uprobes.c
> > > > > > > @@ -39,7 +39,7 @@ static struct rb_root uprobes_tree = RB_ROOT;
> > > > > > > */
> > > > > > > #define no_uprobe_events() RB_EMPTY_ROOT(&uprobes_tree)
> > > > > > >
> > > > > > > -static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */
> > > > > > > +static DEFINE_RWLOCK(uprobes_treelock); /* serialize rbtree access */
> > > > > > >
> > > > > > > #define UPROBES_HASH_SZ 13
> > > > > > > /* serialize uprobe->pending_list */
> > > > > > > @@ -669,9 +669,9 @@ static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
> > > > > > > {
> > > > > > > struct uprobe *uprobe;
> > > > > > >
> > > > > > > - spin_lock(&uprobes_treelock);
> > > > > > > + read_lock(&uprobes_treelock);
> > > > > > > uprobe = __find_uprobe(inode, offset);
> > > > > > > - spin_unlock(&uprobes_treelock);
> > > > > > > + read_unlock(&uprobes_treelock);
> > > > > > >
> > > > > > > return uprobe;
> > > > > > > }
> > > > > > > @@ -701,9 +701,9 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
> > > > > > > {
> > > > > > > struct uprobe *u;
> > > > > > >
> > > > > > > - spin_lock(&uprobes_treelock);
> > > > > > > + write_lock(&uprobes_treelock);
> > > > > > > u = __insert_uprobe(uprobe);
> > > > > > > - spin_unlock(&uprobes_treelock);
> > > > > > > + write_unlock(&uprobes_treelock);
> > > > > > >
> > > > > > > return u;
> > > > > > > }
> > > > > > > @@ -935,9 +935,9 @@ static void delete_uprobe(struct uprobe *uprobe)
> > > > > > > if (WARN_ON(!uprobe_is_active(uprobe)))
> > > > > > > return;
> > > > > > >
> > > > > > > - spin_lock(&uprobes_treelock);
> > > > > > > + write_lock(&uprobes_treelock);
> > > > > > > rb_erase(&uprobe->rb_node, &uprobes_tree);
> > > > > > > - spin_unlock(&uprobes_treelock);
> > > > > > > + write_unlock(&uprobes_treelock);
> > > > > > > RB_CLEAR_NODE(&uprobe->rb_node); /* for uprobe_is_active() */
> > > > > > > put_uprobe(uprobe);
> > > > > > > }
> > > > > > > @@ -1298,7 +1298,7 @@ static void build_probe_list(struct inode *inode,
> > > > > > > min = vaddr_to_offset(vma, start);
> > > > > > > max = min + (end - start) - 1;
> > > > > > >
> > > > > > > - spin_lock(&uprobes_treelock);
> > > > > > > + read_lock(&uprobes_treelock);
> > > > > > > n = find_node_in_range(inode, min, max);
> > > > > > > if (n) {
> > > > > > > for (t = n; t; t = rb_prev(t)) {
> > > > > > > @@ -1316,7 +1316,7 @@ static void build_probe_list(struct inode *inode,
> > > > > > > get_uprobe(u);
> > > > > > > }
> > > > > > > }
> > > > > > > - spin_unlock(&uprobes_treelock);
> > > > > > > + read_unlock(&uprobes_treelock);
> > > > > > > }
> > > > > > >
> > > > > > > /* @vma contains reference counter, not the probed instruction. */
> > > > > > > @@ -1407,9 +1407,9 @@ vma_has_uprobes(struct vm_area_struct *vma, unsigned long start, unsigned long e
> > > > > > > min = vaddr_to_offset(vma, start);
> > > > > > > max = min + (end - start) - 1;
> > > > > > >
> > > > > > > - spin_lock(&uprobes_treelock);
> > > > > > > + read_lock(&uprobes_treelock);
> > > > > > > n = find_node_in_range(inode, min, max);
> > > > > > > - spin_unlock(&uprobes_treelock);
> > > > > > > + read_unlock(&uprobes_treelock);
> > > > > > >
> > > > > > > return !!n;
> > > > > > > }
> > > > > > > --
> > > > > > > 2.43.0
> > > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > > >
> > > >
> > > > --
> > > > Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@...nel.org>
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