Re: [PATCH v2 11/11] perf/uprobe: Add uretprobe timer

Peter Zijlstra
2024-07-15T11:41:13Z

On Fri, Jul 12, 2024 at 02:43:52PM -0700, Andrii Nakryiko wrote:
> + bpf
>
> On Thu, Jul 11, 2024 at 4:07 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > In order to put a bound on the uretprobe_srcu critical section, add a
> > timer to uprobe_task. Upon every RI added or removed the timer is
> > pushed forward to now + 1s. If the timer were ever to fire, it would
> > convert the SRCU 'reference' to a refcount reference if possible.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > include/linux/uprobes.h | 8 +++++
> > kernel/events/uprobes.c | 67 ++++++++++++++++++++++++++++++++++++++++++++----
> > 2 files changed, 69 insertions(+), 6 deletions(-)
> >
> > --- a/include/linux/uprobes.h
> > +++ b/include/linux/uprobes.h
> > @@ -15,6 +15,7 @@
> > #include <linux/rbtree.h>
> > #include <linux/types.h>
> > #include <linux/wait.h>
> > +#include <linux/timer.h>
> >
> > struct vm_area_struct;
> > struct mm_struct;
> > @@ -79,6 +80,10 @@ struct uprobe_task {
> > struct return_instance *return_instances;
> > unsigned int depth;
> > unsigned int active_srcu_idx;
> > +
> > + struct timer_list ri_timer;
> > + struct callback_head ri_task_work;
> > + struct task_struct *task;
> > };
> >
> > struct return_instance {
> > @@ -86,7 +91,8 @@ struct return_instance {
> > unsigned long func;
> > unsigned long stack; /* stack pointer */
> > unsigned long orig_ret_vaddr; /* original return address */
> > - bool chained; /* true, if instance is nested */
> > + u8 chained; /* true, if instance is nested */
> > + u8 has_ref;
>
> Why bool -> u8 switch? You don't touch chained, so why change its
> type? And for has_ref you interchangeably use 0 and true for the same
> field. Let's stick to bool as there is nothing wrong with it?

sizeof(_Bool) is implementation defined. It is 1 for x86_64, but there
are platforms where it ends up begin 4 (some PowerPC ABIs among others.
I didn't want to grow this structure for no reason.

> > int srcu_idx;
> >
> > struct return_instance *next; /* keep as stack */
>
> [...]
>
> > @@ -1822,13 +1864,20 @@ static int dup_utask(struct task_struct
> > return -ENOMEM;
> >
> > *n = *o;
> > - __srcu_clone_read_lock(&uretprobes_srcu, n->srcu_idx);
> > + if (n->uprobe) {
> > + if (n->has_ref)
> > + get_uprobe(n->uprobe);
> > + else
> > + __srcu_clone_read_lock(&uretprobes_srcu, n->srcu_idx);
> > + }
> > n->next = NULL;
> >
> > *p = n;
> > p = &n->next;
> > n_utask->depth++;
> > }
> > + if (n_utask->return_instances)
> > + mod_timer(&n_utask->ri_timer, jiffies + HZ);
>
> let's add #define for HZ, so it's adjusted in just one place (instead
> of 3 as it is right now)

Can do I suppose.

> Also, we can have up to 64 levels of uretprobe nesting, so,
> technically, the user can cause a delay of 64 seconds in total. Maybe
> let's use something smaller than a full second? After all, if the
> user-space function has high latency, then this refcount congestion is
> much less of a problem. I'd set it to something like 50-100 ms for
> starters.

Before you know it we'll have a sysctl :/ But sure, we can do something
shorter.