| Message ID | 20241010205644.3831427-5-andrii@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | uprobes,mm: speculative lockless VMA-to-uprobe lookup | expand |
On 10/10, Andrii Nakryiko wrote: > > Suggested-by: Matthew Wilcox <willy@infradead.org> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org> > --- > kernel/events/uprobes.c | 50 +++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 50 insertions(+) FWIW, Reviewed-by: Oleg Nesterov <oleg@redhat.com>
On Thu, Oct 10, 2024 at 01:56:44PM -0700, Andrii Nakryiko wrote: > Suggested-by: Matthew Wilcox <willy@infradead.org> I'm fairly sure I've suggested much the same :-) > Signed-off-by: Andrii Nakryiko <andrii@kernel.org> > --- > kernel/events/uprobes.c | 50 +++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 50 insertions(+) > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c > index fa1024aad6c4..9dc6e78975c9 100644 > --- a/kernel/events/uprobes.c > +++ b/kernel/events/uprobes.c > @@ -2047,6 +2047,52 @@ 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; > + struct inode *vm_inode; > + unsigned long vm_pgoff, vm_start; > + loff_t offset; > + long seq; > + > + guard(rcu)(); > + > + if (!mmap_lock_speculation_start(mm, &seq)) > + return NULL; So traditional seqcount assumed non-preemptible lock sides and would spin-wait for the LSB to clear, but for PREEMPT_RT we added preemptible seqcount support and that takes the lock to wait, which in this case is exactly the same as returning NULL and doing the lookup holding mmap_lock, so yeah. > + > + vma = vma_lookup(mm, bp_vaddr); > + if (!vma) > + return NULL; > + > + /* vm_file memory can be reused for another instance of struct file, Comment style nit. > + * 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 > + */ > + vm_file = READ_ONCE(vma->vm_file); > + if (!vm_file) > + return NULL; > + > + vm_pgoff = data_race(vma->vm_pgoff); > + vm_start = data_race(vma->vm_start); > + vm_inode = data_race(vm_file->f_inode); So... seqcount has kcsan annotations other than data_race(). I suppose this works, but it all feels like a bad copy with random changes. > + > + offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start); > + uprobe = find_uprobe_rcu(vm_inode, offset); > + if (!uprobe) > + return NULL; > + > + /* now double check that nothing about MM changed */ > + if (!mmap_lock_speculation_end(mm, seq)) > + return NULL; Typically seqcount does a re-try here. > + > + return uprobe; > +}
On Wed, Oct 23, 2024 at 12:22 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, Oct 10, 2024 at 01:56:44PM -0700, Andrii Nakryiko wrote: > > > Suggested-by: Matthew Wilcox <willy@infradead.org> > > I'm fairly sure I've suggested much the same :-) I'll add another Suggested-by, didn't mean to rob anyone of credits :) > > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org> > > --- > > kernel/events/uprobes.c | 50 +++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 50 insertions(+) > > > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c > > index fa1024aad6c4..9dc6e78975c9 100644 > > --- a/kernel/events/uprobes.c > > +++ b/kernel/events/uprobes.c > > @@ -2047,6 +2047,52 @@ 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; > > + struct inode *vm_inode; > > + unsigned long vm_pgoff, vm_start; > > + loff_t offset; > > + long seq; > > + > > + guard(rcu)(); > > + > > + if (!mmap_lock_speculation_start(mm, &seq)) > > + return NULL; > > So traditional seqcount assumed non-preemptible lock sides and would > spin-wait for the LSB to clear, but for PREEMPT_RT we added preemptible > seqcount support and that takes the lock to wait, which in this case is > exactly the same as returning NULL and doing the lookup holding > mmap_lock, so yeah. > yep, and on configurations with CONFIG_PER_VMA_LOCK=n this will always return false > > + > > + vma = vma_lookup(mm, bp_vaddr); > > + if (!vma) > > + return NULL; > > + > > + /* vm_file memory can be reused for another instance of struct file, > > Comment style nit. mechanical memory, sorry, missed this one > > > + * 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 > > + */ > > + vm_file = READ_ONCE(vma->vm_file); > > + if (!vm_file) > > + return NULL; > > + > > + vm_pgoff = data_race(vma->vm_pgoff); > > + vm_start = data_race(vma->vm_start); > > + vm_inode = data_race(vm_file->f_inode); > > So... seqcount has kcsan annotations other than data_race(). I suppose > this works, but it all feels like a bad copy with random changes. I'm not sure what this means... Do I need to change anything? Drop data_race()? Use READ_ONCE()? Do nothing? > > > + > > + offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start); > > + uprobe = find_uprobe_rcu(vm_inode, offset); > > + if (!uprobe) > > + return NULL; > > + > > + /* now double check that nothing about MM changed */ > > + if (!mmap_lock_speculation_end(mm, seq)) > > + return NULL; > > Typically seqcount does a re-try here. I'd like to keep it simple, we have fallback to locked version in case of a race > > > + > > + return uprobe; > > +}
On Wed, Oct 23, 2024 at 01:02:53PM -0700, Andrii Nakryiko wrote: > > > + * 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 > > > + */ > > > + vm_file = READ_ONCE(vma->vm_file); > > > + if (!vm_file) > > > + return NULL; > > > + > > > + vm_pgoff = data_race(vma->vm_pgoff); > > > + vm_start = data_race(vma->vm_start); > > > + vm_inode = data_race(vm_file->f_inode); > > > > So... seqcount has kcsan annotations other than data_race(). I suppose > > this works, but it all feels like a bad copy with random changes. > > I'm not sure what this means... Do I need to change anything? Drop > data_race()? Use READ_ONCE()? Do nothing? Keep for now. I've ranted at 1/n a bit, but unless the response is: yeah, obviously this should be seqcount (unlikely) this is something that can be fixed later (*sigh* always later... this todo list is a problem).
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index fa1024aad6c4..9dc6e78975c9 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c @@ -2047,6 +2047,52 @@ 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; + struct inode *vm_inode; + unsigned long vm_pgoff, vm_start; + loff_t offset; + long seq; + + guard(rcu)(); + + if (!mmap_lock_speculation_start(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 + */ + vm_file = READ_ONCE(vma->vm_file); + if (!vm_file) + return NULL; + + vm_pgoff = data_race(vma->vm_pgoff); + vm_start = data_race(vma->vm_start); + vm_inode = data_race(vm_file->f_inode); + + offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start); + uprobe = find_uprobe_rcu(vm_inode, offset); + 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) { @@ -2054,6 +2100,10 @@ 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) {
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_{start,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). Suggested-by: Matthew Wilcox <willy@infradead.org> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> --- kernel/events/uprobes.c | 50 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+)