Re: [PATCH v4 tip/perf/core 4/4] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

Suren Baghdasaryan
2024-11-12T01:05:06Z

On Mon, Nov 11, 2024 at 4:28 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Sun, 27 Oct 2024 18:08:18 -0700
> Andrii Nakryiko <andrii@kernel.org> wrote:
>
> > Given filp_cachep is marked SLAB_TYPESAFE_BY_RCU (and FMODE_BACKING
> > files, a special case, now goes through RCU-delated freeing), we can
> > safely access vma->vm_file->f_inode field locklessly under just
> > rcu_read_lock() protection, which enables looking up uprobe from
> > uprobes_tree completely locklessly and speculatively without the need to
> > acquire mmap_lock for reads. In most cases, anyway, assuming that there
> > are no parallel mm and/or VMA modifications. The underlying struct
> > file's memory won't go away from under us (even if struct file can be
> > reused in the meantime).
> >
> > We rely on newly added mmap_lock_speculation_{begin,end}() helpers to
> > validate that mm_struct stays intact for entire duration of this
> > speculation. If not, we fall back to mmap_lock-protected lookup.
> > The speculative logic is written in such a way that it will safely
> > handle any garbage values that might be read from vma or file structs.
> >
> > Benchmarking results speak for themselves.
> >
> > BEFORE (latest tip/perf/core)
> > =============================
> > uprobe-nop ( 1 cpus): 3.384 ± 0.004M/s ( 3.384M/s/cpu)
> > uprobe-nop ( 2 cpus): 5.456 ± 0.005M/s ( 2.728M/s/cpu)
> > uprobe-nop ( 3 cpus): 7.863 ± 0.015M/s ( 2.621M/s/cpu)
> > uprobe-nop ( 4 cpus): 9.442 ± 0.008M/s ( 2.360M/s/cpu)
> > uprobe-nop ( 5 cpus): 11.036 ± 0.013M/s ( 2.207M/s/cpu)
> > uprobe-nop ( 6 cpus): 10.884 ± 0.019M/s ( 1.814M/s/cpu)
> > uprobe-nop ( 7 cpus): 7.897 ± 0.145M/s ( 1.128M/s/cpu)
> > uprobe-nop ( 8 cpus): 10.021 ± 0.128M/s ( 1.253M/s/cpu)
> > uprobe-nop (10 cpus): 9.932 ± 0.170M/s ( 0.993M/s/cpu)
> > uprobe-nop (12 cpus): 8.369 ± 0.056M/s ( 0.697M/s/cpu)
> > uprobe-nop (14 cpus): 8.678 ± 0.017M/s ( 0.620M/s/cpu)
> > uprobe-nop (16 cpus): 7.392 ± 0.003M/s ( 0.462M/s/cpu)
> > uprobe-nop (24 cpus): 5.326 ± 0.178M/s ( 0.222M/s/cpu)
> > uprobe-nop (32 cpus): 5.426 ± 0.059M/s ( 0.170M/s/cpu)
> > uprobe-nop (40 cpus): 5.262 ± 0.070M/s ( 0.132M/s/cpu)
> > uprobe-nop (48 cpus): 6.121 ± 0.010M/s ( 0.128M/s/cpu)
> > uprobe-nop (56 cpus): 6.252 ± 0.035M/s ( 0.112M/s/cpu)
> > uprobe-nop (64 cpus): 7.644 ± 0.023M/s ( 0.119M/s/cpu)
> > uprobe-nop (72 cpus): 7.781 ± 0.001M/s ( 0.108M/s/cpu)
> > uprobe-nop (80 cpus): 8.992 ± 0.048M/s ( 0.112M/s/cpu)
> >
> > AFTER
> > =====
> > uprobe-nop ( 1 cpus): 3.534 ± 0.033M/s ( 3.534M/s/cpu)
> > uprobe-nop ( 2 cpus): 6.701 ± 0.007M/s ( 3.351M/s/cpu)
> > uprobe-nop ( 3 cpus): 10.031 ± 0.007M/s ( 3.344M/s/cpu)
> > uprobe-nop ( 4 cpus): 13.003 ± 0.012M/s ( 3.251M/s/cpu)
> > uprobe-nop ( 5 cpus): 16.274 ± 0.006M/s ( 3.255M/s/cpu)
> > uprobe-nop ( 6 cpus): 19.563 ± 0.024M/s ( 3.261M/s/cpu)
> > uprobe-nop ( 7 cpus): 22.696 ± 0.054M/s ( 3.242M/s/cpu)
> > uprobe-nop ( 8 cpus): 24.534 ± 0.010M/s ( 3.067M/s/cpu)
> > uprobe-nop (10 cpus): 30.475 ± 0.117M/s ( 3.047M/s/cpu)
> > uprobe-nop (12 cpus): 33.371 ± 0.017M/s ( 2.781M/s/cpu)
> > uprobe-nop (14 cpus): 38.864 ± 0.004M/s ( 2.776M/s/cpu)
> > uprobe-nop (16 cpus): 41.476 ± 0.020M/s ( 2.592M/s/cpu)
> > uprobe-nop (24 cpus): 64.696 ± 0.021M/s ( 2.696M/s/cpu)
> > uprobe-nop (32 cpus): 85.054 ± 0.027M/s ( 2.658M/s/cpu)
> > uprobe-nop (40 cpus): 101.979 ± 0.032M/s ( 2.549M/s/cpu)
> > uprobe-nop (48 cpus): 110.518 ± 0.056M/s ( 2.302M/s/cpu)
> > uprobe-nop (56 cpus): 117.737 ± 0.020M/s ( 2.102M/s/cpu)
> > uprobe-nop (64 cpus): 124.613 ± 0.079M/s ( 1.947M/s/cpu)
> > uprobe-nop (72 cpus): 133.239 ± 0.032M/s ( 1.851M/s/cpu)
> > uprobe-nop (80 cpus): 142.037 ± 0.138M/s ( 1.775M/s/cpu)
> >
> > Previously total throughput was maxing out at 11mln/s, and gradually
> > declining past 8 cores. With this change, it now keeps growing with each
> > added CPU, reaching 142mln/s at 80 CPUs (this was measured on a 80-core
> > Intel(R) Xeon(R) Gold 6138 CPU @ 2.00GHz).
> >
>
> Looks good to me, except one question below.
>
> > Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> > Suggested-by: Matthew Wilcox <willy@infradead.org>
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> > kernel/events/uprobes.c | 45 +++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 45 insertions(+)
> >
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 290c445768fa..efcd62f7051d 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -2074,6 +2074,47 @@ static int is_trap_at_addr(struct mm_struct *mm, unsigned long vaddr)
> > return is_trap_insn(&opcode);
> > }
> >
> > +static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
> > +{
> > + struct mm_struct *mm = current->mm;
> > + struct uprobe *uprobe = NULL;
> > + struct vm_area_struct *vma;
> > + struct file *vm_file;
> > + loff_t offset;
> > + unsigned int seq;
> > +
> > + guard(rcu)();
> > +
> > + if (!mmap_lock_speculation_begin(mm, &seq))
> > + return NULL;
> > +
> > + vma = vma_lookup(mm, bp_vaddr);
> > + if (!vma)
> > + return NULL;
> > +
> > + /*
> > + * vm_file memory can be reused for another instance of struct file,
> > + * but can't be freed from under us, so it's safe to read fields from
> > + * it, even if the values are some garbage values; ultimately
> > + * find_uprobe_rcu() + mmap_lock_speculation_end() check will ensure
> > + * that whatever we speculatively found is correct
>
> If vm_file is a garbage value, may `vm_file->f_inode` access be dangerous?
>
> > + */
> > + vm_file = READ_ONCE(vma->vm_file);
> > + if (!vm_file)
> > + return NULL;
> > +
> > + offset = (loff_t)(vma->vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vma->vm_start);
> > + uprobe = find_uprobe_rcu(vm_file->f_inode, offset);
> ^^^^ Here
>
> if it only stores vm_file or NULL, there's no problem.

IIRC correctly, vma->vm_file is RCU-safe and we are in the read RCU
section, so it should not contain a garbage value.

>
> Thank you,
>
> > + if (!uprobe)
> > + return NULL;
> > +
> > + /* now double check that nothing about MM changed */
> > + if (!mmap_lock_speculation_end(mm, seq))
> > + return NULL;
> > +
> > + return uprobe;
> > +}
> > +
> > /* assumes being inside RCU protected region */
> > static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swbp)
> > {
> > @@ -2081,6 +2122,10 @@ static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swb
> > struct uprobe *uprobe = NULL;
> > struct vm_area_struct *vma;
> >
> > + uprobe = find_active_uprobe_speculative(bp_vaddr);
> > + if (uprobe)
> > + return uprobe;
> > +
> > mmap_read_lock(mm);
> > vma = vma_lookup(mm, bp_vaddr);
> > if (vma) {
> > --
> > 2.43.5
> >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@kernel.org>