Re: [PATCH] uprobes: reduce contention on uprobes_tree access

[Ingo Molnar <mingo@kernel.org>]

* Jonathan Haslam <jonathan.haslam@gmail.com> 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.
> 
> 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).

Have you considered/measured per-CPU RW semaphores?

Thanks,

	Ingo