[Bug runtime/31014] New: Uprobes registered in task finder would block the target processes for long time

[agentzh at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31014

            Bug ID: 31014
           Summary: Uprobes registered in task finder would block the
                    target processes for long time
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: runtime
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

Currently, the stap runtime tries to call uprobe_register() to register new
uprobes inside the context of the target processes via the task finder. This
makes little sense for inode-based uprobes implemented in modern kernels and
also introduces significant (8ms+) delay in all the target processes via the
task work mechanism. Such delays may manifest themselves in syscalls like
epoll_wait inside the target processes.

All the target processes are affected even though only one task finder callback
gets to call uprobe_register(). Other concurrent target processes would just
wait on the lock `c->consumer_lock`.

After talking with fche, I'd propose that we should register uprobes early
before starting the task finder callbacks. This still may block a CPU core for
8ms+ or so, but it no longer blocks all the target processes in their own
process contexts.

Will prepare a patch shortly.

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