Re: [PATCH v3] ssdd: mitigate tracee starvation

[John Kacur <jkacur@redhat.com>]

On Fri, 19 Sep 2025, Derek Barbosa wrote:

> When ssdd is invoked with nforks > 100 && niters == 10000 on a tuned,
> realtime kernel, the following error messages can be seen:
> 
> EXITING, ERROR: wait on PTRACE_SINGLESTEP #385: no SIGCHLD seen (signal count == 0), signo 5
> EXITING, ERROR: wait on PTRACE_SINGLESTEP #398: no SIGCHLD seen (signal count == 0), signo 5
> EXITING, ERROR: wait on PTRACE_SINGLESTEP #385: no SIGCHLD seen (signal count == 0), signo 5
> ...
> 
> This behavior is caused by ptrace_stop() being unable to sleep after
> taking tasklist_lock().
> 
> As forktest() generates "niter" PTRACE_SINGLESTEP's for nforks, in the
> rare event where nforks exceeds the defaults by a large order of
> magnitude, the sporadic test failures caused by missing SIGCHLDs
> indicates that the tracees are unable to effectively wait for their
> asynchronous signals to arrive --as denoted in the previous sleeps for
> check_sigchld().
> 
> Therefore, by performing an sigtimedwait() in check_sigchld(), we
> give the tracee enough CPU time to call
> do_notify_parent_cldstop()->send_signal_locked().
> 
> The observed behavior after appling this patch mitigates the
> aforementioned issue in scenarios with a high number of nforks.
> 
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Suggested-by: Crystal Wood <crwood@redhat.com>
> Signed-off-by: Derek Barbosa <debarbos@redhat.com>
> ---
> V1 -> V2: Addressed review comments, removed usleep() in favor of
> sigtimedwait().
> V2 -> V3: Addressed checkpatch.pl complaints.
> 
>  src/ssdd/ssdd.c | 65 ++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 48 insertions(+), 17 deletions(-)
> 

Signed-off-by: John Kacur <jkacur@redhat.com>