On Fri, 17 Jul 2026 10:32:52 -0700 Usama Arif <[email protected]> wrote:
> trace_user_fault_read() samples nr_context_switches_cpu() before enabling > preemption and retries the user copy if the counter changes. The helper > returns unsigned long long because rq->nr_switches is u64, but the saved > value is unsigned int. > > Once a CPU has performed 2^32 context switches, assigning the counter to > cnt discards its upper bits. The comparison after the copy promotes cnt > back to unsigned long long, but the lost bits remain zero, so it reports a > change even when the task was never scheduled out. Every retry then fails > the same way until the 100-try guard warns and the user copy is abandoned. > > This affects long-running systems and workloads with high context-switch > rates. A CPU switching 1,000 times per second takes about 50 days. > > Store the sampled count in unsigned long long so the full value is > preserved. Looks good to me. Acked-by: Masami Hiramatsu (Google) <[email protected]> Thanks! > > Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read > user space") > Cc: [email protected] > Reported-by: Breno Leitao <[email protected]> > Signed-off-by: Usama Arif <[email protected]> > --- > kernel/trace/trace.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 1146b83b711a..412a8daf2162 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -6188,7 +6188,7 @@ char *trace_user_fault_read(struct trace_user_buf_info > *tinfo, > { > int cpu = smp_processor_id(); > char *buffer = per_cpu_ptr(tinfo->tbuf, cpu)->buf; > - unsigned int cnt; > + unsigned long long cnt; > int trys = 0; > int ret; > > -- > 2.53.0-Meta > -- Masami Hiramatsu (Google) <[email protected]>
