On Sun, 27 Oct 2024 18:08:18 -0700
Andrii Nakryiko <andrii@xxxxxxxxxx> 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@xxxxxxxxxx>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
> 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.
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@xxxxxxxxxx>
