On Wed, Mar 27, 2024 at 5:45 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Wed, Mar 27, 2024 at 5:18 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > On Wed, 27 Mar 2024 17:06:01 +0000 > > Jonthan Haslam <jonathan.haslam@xxxxxxxxx> 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@xxxxxxxxx> > > > > > > > --- > > > > > > > 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@xxxxxxxxxx> > > > > > > > > > > > > -- > > > > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> > > > > > > -- > > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
