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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 12 Nov 2024 10:09:58 -0800
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Mon, Nov 11, 2024 at 5:05 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > 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.
> 
> Correct. vm_file itself can be either TYPESAFE_BY_RCU for normal
> files, or properly RCU protected for FMODE_BACKING ones. Either way,
> there is some correct struct file pointed to, and so all this is valid
> and won't dereference invalid memory.

OK, thanks for confirmation! This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> 
> >
> > >
> > > 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>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>