On Thu, Mar 21, 2024 at 7:57 AM Jonathan Haslam <jonathan.haslam@xxxxxxxxx> wrote: > > Active uprobes are stored in an RB tree and accesses to this tree are > dominated by read operations. Currently these accesses are serialized by > a spinlock but this leads to enormous contention when large numbers of > threads are executing active probes. > > This patch converts the spinlock used to serialize access to the > uprobes_tree RB tree into a reader-writer spinlock. This lock type > aligns naturally with the overwhelmingly read-only nature of the tree > usage here. Although the addition of reader-writer spinlocks are > discouraged [0], this fix is proposed as an interim solution while an > RCU based approach is implemented (that work is in a nascent form). This > fix also has the benefit of being trivial, self contained and therefore > simple to backport. Yep, makes sense, I think we'll want to backport this ASAP to some of the old kernels we have. Thanks! Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > This change has been tested against production workloads that exhibit > significant contention on the spinlock and an almost order of magnitude > reduction for mean uprobe execution time is observed (28 -> 3.5 microsecs). > > [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 >
