On Fri, 10 Jul 2026 08:33:57 -0400 Steven Rostedt <[email protected]> wrote:
> On Fri, 10 Jul 2026 12:22:31 +0900 > Masami Hiramatsu (Google) <[email protected]> wrote: > > > Hm, in my view, this warning indicates that the circuit breaker has > > triggered correctly, so that is not a bug. Under the heavy memory > > pressure and low-memory situation, the page can be reclaimed soon > > after it is copied. > > So you are saying that every time the copy_from_user() is executed, the > page is reclaimed? And this causes a schedule? > > Now, I did have a version that used sched_switch and only incremented the > counter when a non-kernel thread was scheduled in. Then the test would > check if the counter increased by 2 or more. As an increase by 1 meant that > only kernel threads scheduled in which would not corrupt the buffer. The 1 > increment was the current task scheduling back. > > This is based on that work (I'm glad I save old versions in my git tree :-) > > Funny, the comments were from the original change I did back in August of > 2025, which mentions kernel threads scheduling in to handle the fault. > > I also kept this around in case it was needed. Looks like it may be needed. Hmm, it's not clear whether this is the result of a scheduling storm caused by kernel threads. But maybe worth to try. Jeongho, can you try this patch? Or/and you maybe also try using mlock the LMKD process memory which is passed to trace_marker. Thank you, > > -- Steve > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 18710c190c92..19354fe2fca1 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -53,6 +53,8 @@ > #include <linux/io.h> /* vmap_page_range() */ > #include <linux/fs_context.h> > > +#include <trace/events/sched.h> > + > #include <asm/setup.h> /* COMMAND_LINE_SIZE */ > > #include "trace.h" > @@ -5984,6 +5986,32 @@ struct trace_user_buf { > static DEFINE_MUTEX(trace_user_buffer_mutex); > static struct trace_user_buf_info *trace_user_buffer; > > +static DEFINE_PER_CPU(unsigned long, sched_switch_cnt); > + > +/* > + * The per CPU buffer trace_user_buffer is written to optimstically. > + * The counter sched_switch_cnt is taken, preemption is enabled, > + * the copying of the user space memory is placed into the trace_user_buffer, > + * Preeption is re-enabled and the count is read again. If the count is > greater > + * than one from its previous reading, it means that another user space > + * task scheduled in and the buffer is unreliable for use. > + */ > +static void > +probe_sched_switch(void *ignore, bool preempt, > + struct task_struct *prev, struct task_struct *next, > + unsigned int prev_state) > +{ > + /* > + * The buffer can only be corrupted by another user space task. > + * Ignore kernel tasks that may be scheduled in order to process > + * the faulting memory. > + */ > + if (!is_user_task(next)) > + return; > + > + this_cpu_inc(sched_switch_cnt); > +} > + > /** > * trace_user_fault_destroy - free up allocated memory of a trace user buffer > * @tinfo: The descriptor to free up > @@ -6003,6 +6031,8 @@ void trace_user_fault_destroy(struct > trace_user_buf_info *tinfo) > kfree(buf); > } > free_percpu(tinfo->tbuf); > + > + unregister_trace_sched_switch(probe_sched_switch, NULL); > } > > static int user_fault_buffer_enable(struct trace_user_buf_info *tinfo, > size_t size) > @@ -6053,11 +6083,17 @@ static int user_buffer_init(struct > trace_user_buf_info **tinfo, size_t size) > > lockdep_assert_held(&trace_user_buffer_mutex); > > + ret = register_trace_sched_switch(probe_sched_switch, NULL); > + if (ret < 0) > + return ret; > + > if (!*tinfo) { > alloc = true; > *tinfo = kzalloc_obj(**tinfo); > - if (!*tinfo) > + if (!*tinfo) { > + unregister_trace_sched_switch(probe_sched_switch, NULL); > return -ENOMEM; > + } > } > > ret = user_fault_buffer_enable(*tinfo, size); > @@ -6241,7 +6277,7 @@ char *trace_user_fault_read(struct trace_user_buf_info > *tinfo, > return NULL; > > /* Read the current CPU context switch counter */ > - cnt = nr_context_switches_cpu(cpu); > + cnt = this_cpu_read(sched_switch_cnt); > > /* > * Preemption is going to be enabled, but this task must > @@ -6272,12 +6308,19 @@ char *trace_user_fault_read(struct > trace_user_buf_info *tinfo, > return NULL; > > /* > - * Preemption is disabled again, now check the per CPU context > - * switch counter. If it doesn't match, then another user space > - * process may have schedule in and corrupted our buffer. In > that > - * case the copying must be retried. > + * Preemption is disabled again, now check the sched_switch_cnt. > + * If it increased by two or more, then another user space > process > + * may have schedule in and corrupted our buffer. In that case > + * the copying must be retried. > + * > + * Note, if this task was scheduled out and only kernel threads > + * were scheduled in (maybe to process the fault), then the > + * counter would increment again when this task scheduled in. > + * If this task scheduled out and another user task scheduled > + * in, this task would still need to be scheduled back in and > + * the counter would increment by at least two. > */ > - } while (nr_context_switches_cpu(cpu) != cnt); > + } while (this_cpu_read(sched_switch_cnt) > cnt + 1); > > return buffer; > } -- Masami Hiramatsu (Google) <[email protected]>
