Hi Steve, On Tue, 2023-04-11 at 12:44 -0400, Steven Rostedt wrote: > External email : Please do not click links or open attachments until > you have verified the sender or the content. > > > Please have each new patch be a new thread, and not a Cc to the > previous > version of the patch. As it makes it hard to find in INBOXs. > No problem, got it. > On Mon, 10 Apr 2023 15:35:08 +0800 > Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx> wrote: > > > Write to buffer_size_kb can permanently fail, due to > > cpu_online_mask may > > changed between two for_each_online_buffer_cpu loops. > > The number of increasing and decreasing on cpu_buffer- > > >resize_disable > > may be inconsistent, leading that the resize_disabled in some CPUs > > becoming none zero after ring_buffer_reset_online_cpus return. > > > > This issue can be reproduced by "echo 0 > trace" while hotplugging > > cpu. > > After reproducing success, we can find out buffer_size_kb will not > > be > > functional anymore. > > > > Prevent the two "loops" in this function from iterating through > > different > > online cpus by copying cpu_online_mask at the entry of the > > function. > > > > The "Changes from" need to go below the '---', otherwise they are > added to > the git commit (we don't want it there). > Will remember this, won't happened next time :) > > Changes from v1 to v3: > > Declare the cpumask variable statically rather than dynamically. > > > > Changes from v2 to v3: > > Considering holding cpu_hotplug_lock too long because of the > > synchronize_rcu(), maybe it's better to prevent the issue by > > copying > > cpu_online_mask at the entry of the function as V1 does, instead > > of > > using cpus_read_lock(). > > > > Link: > > https://lore.kernel.org/lkml/20230408052226.25268-1-Tze-nan.Wu@xxxxxxxxxxxx/ > > Link: > > https://lore.kernel.org/oe-kbuild-all/202304082051.Dp50upfS-lkp@xxxxxxxxx/ > > Link: > > https://lore.kernel.org/oe-kbuild-all/202304081615.eiaqpbV8-lkp@xxxxxxxxx/ > > > > Cc: stable@xxxxxxxxxxxxxxx > > Cc: npiggin@xxxxxxxxx > > Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by > > avoiding synchronize_rcu for each CPU") > > Reported-by: kernel test robot <lkp@xxxxxxxxx> > > Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx> > > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx> > > --- > > This is where the "Changes from" go. And since this patch is not > suppose to > be a Cc. But since it's still good to have a link to it. You could > do: > > Changes from v2 to v3: > https://lore.kernel.org/linux-trace-kernel/20230409024616.31099-1-Tze-nan.Wu@xxxxxxxxxxxx/ > Considering holding cpu_hotplug_lock too long because of the > synchronize_rcu(), maybe it's better to prevent the issue by > copying > cpu_online_mask at the entry of the function as V1 does, instead of > using cpus_read_lock(). > > > Where the previous version changes has the lore link to the previous > patch, > in case someone wants to look at it. > Sure, a link here is really helpful. Will follow this format in the future. > > > kernel/trace/ring_buffer.c | 14 +++++++++----- > > 1 file changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/trace/ring_buffer.c > > b/kernel/trace/ring_buffer.c > > index 76a2d91eecad..dc758930dacb 100644 > > --- a/kernel/trace/ring_buffer.c > > +++ b/kernel/trace/ring_buffer.c > > @@ -288,9 +288,6 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data); > > #define for_each_buffer_cpu(buffer, cpu) \ > > for_each_cpu(cpu, buffer->cpumask) > > > > -#define for_each_online_buffer_cpu(buffer, cpu) \ > > - for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask) > > - > > #define TS_SHIFT 27 > > #define TS_MASK ((1ULL << TS_SHIFT) - 1) > > #define TS_DELTA_TEST (~TS_MASK) > > @@ -5353,12 +5350,19 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); > > void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) > > { > > struct ring_buffer_per_cpu *cpu_buffer; > > + cpumask_t reset_online_cpumask; > > It's usually considered bad form to put a cpumask on the stack. As it > can > be 128 bytes for a machine with 1024 CPUs (and yes they do exist). > Also, > the mask size is set to NR_CPUS not the actual size, so you do not > even > need to have it that big. > Never thought about that until you told me, I will keep it in mind before declare a cpumask next time. > > > int cpu; > > > > + /* > > + * Record cpu_online_mask here to make sure we iterate > > through the same > > + * online CPUs in the following two loops. > > + */ > > + cpumask_copy(&reset_online_cpumask, cpu_online_mask); > > + > > /* prevent another thread from changing buffer sizes */ > > mutex_lock(&buffer->mutex); > > > > - for_each_online_buffer_cpu(buffer, cpu) { > > + for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) > > { > > cpu_buffer = buffer->buffers[cpu]; > > > > atomic_inc(&cpu_buffer->resize_disabled); > > Anyway, we don't need to modify any of the above, and just do the > following > instead of atomic_inc(): > > #define RESET_BIT (1 << 30) > > atomic_add(&cpu_buffer->resize_disabled, RESET_BIT); > > > > @@ -5368,7 +5372,7 @@ void ring_buffer_reset_online_cpus(struct > > trace_buffer *buffer) > > /* Make sure all commits have finished */ > > synchronize_rcu(); > > > > - for_each_online_buffer_cpu(buffer, cpu) { > > + for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) Maybe we should use for_each_buffer_cpu(buffer, cpu) here? since a CPU may also came offline during synchronize_rcu(). > > { > > cpu_buffer = buffer->buffers[cpu]; > > Then here we can do: > > /* > * If a CPU came online during the synchronize_rcu(), > then > * ignore it. > */ > if (!atomic_read(&cpu_buffer->resize_disabled) & > RESET_BIT)) > continue; > > atomic_sub(&cpu_buffer->resize_disabled, RESET_BIT); > > > As the resize_disabled only needs to be set to something to make it > disabled. > > -- Steve > Thanks for all your suggestions, learn a lot from here, really appriciate :). I will upload a v4 patch in new thread as soon as the new patch pass my test. -- Tzenan > > > > reset_disabled_cpu_buffer(cpu_buffer); > >