[PATCH v5 1/1] libcamera: add method to set thread affinity

Laurent Pinchart laurent.pinchart at ideasonboard.com
Wed Oct 23 00:49:01 CEST 2024


On Thu, Oct 17, 2024 at 01:59:58PM +0800, Cheng-Hao Yang wrote:
> Hi Laurent,
> 
> I'll update a new version when all changes are applied.

Yes, let's finalize the discussion first :-)

> On Thu, Oct 17, 2024 at 6:08 AM Laurent Pinchart wrote:
> > On Wed, Oct 16, 2024 at 08:43:44AM +0000, Harvey Yang wrote:
> > > From: Han-Lin Chen <hanlinchen at chromium.org>
> > >
> > > Add method to set thread affinity to Thread class.
> > >
> > > Signed-off-by: Han-Lin Chen <hanlinchen at chromium.org>
> > > Co-developed-by: Harvey Yang <chenghaoyang at chromium.org>
> > > Signed-off-by: Harvey Yang <chenghaoyang at chromium.org>
> > > ---
> > >  include/libcamera/base/thread.h |  5 ++++
> > >  src/libcamera/base/thread.cpp   | 46 +++++++++++++++++++++++++++++++++
> > >  2 files changed, 51 insertions(+)
> > >
> > > diff --git a/include/libcamera/base/thread.h b/include/libcamera/base/thread.h
> > > index 4f33de63d..3209d4f7c 100644
> > > --- a/include/libcamera/base/thread.h
> > > +++ b/include/libcamera/base/thread.h
> > > @@ -15,6 +15,7 @@
> > >
> > >  #include <libcamera/base/message.h>
> > >  #include <libcamera/base/signal.h>
> > > +#include <libcamera/base/span.h>
> > >  #include <libcamera/base/utils.h>
> > >
> > >  namespace libcamera {
> > > @@ -35,6 +36,8 @@ public:
> > >       void exit(int code = 0);
> > >       bool wait(utils::duration duration = utils::duration::max());
> > >
> > > +     int setThreadAffinity(const Span<const unsigned int> &cpus);
> > > +
> > >       bool isRunning();
> > >
> > >       Signal<> finished;
> > > @@ -54,6 +57,8 @@ private:
> > >       void startThread();
> > >       void finishThread();
> > >
> > > +     void setThreadAffinityInternal();
> > > +
> > >       void postMessage(std::unique_ptr<Message> msg, Object *receiver);
> > >       void removeMessages(Object *receiver);
> > >
> > > diff --git a/src/libcamera/base/thread.cpp b/src/libcamera/base/thread.cpp
> > > index 8735670b8..5dc787dba 100644
> > > --- a/src/libcamera/base/thread.cpp
> > > +++ b/src/libcamera/base/thread.cpp
> > > @@ -9,6 +9,7 @@
> > >
> > >  #include <atomic>
> > >  #include <list>
> > > +#include <optional>
> > >  #include <sys/syscall.h>
> > >  #include <sys/types.h>
> > >  #include <unistd.h>
> > > @@ -128,6 +129,8 @@ private:
> > >       int exitCode_;
> > >
> > >       MessageQueue messages_;
> > > +
> > > +     std::optional<cpu_set_t> cpuset_ = std::nullopt;
> >
> > Is there a need to initialize this to std::nullopt, as that's what the
> > default constructor does ?
> 
> Removed.
> 
> > >  };
> > >
> > >  /**
> > > @@ -254,6 +257,8 @@ void Thread::start()
> > >       data_->exit_.store(false, std::memory_order_relaxed);
> > >
> > >       thread_ = std::thread(&Thread::startThread, this);
> > > +
> > > +     setThreadAffinityInternal();
> > >  }
> > >
> > >  void Thread::startThread()
> > > @@ -410,6 +415,47 @@ bool Thread::wait(utils::duration duration)
> > >       return hasFinished;
> > >  }
> > >
> > > +/**
> > > + * \brief Set the CPU affinity mask of the thread
> > > + * \param[in] cpus The list of CPU indices that the thread is set affinity to
> > > + *
> > > + * The CPU indices should be within [0, std::thread::hardware_concurrency()).
> > > + * If any index is invalid, no cpu indices will be set.
> >
> >  * If any index is invalid, this function won't modify the thread affinity and
> >  * will return an error.
> 
> Done.
> 
> > > + *
> > > + * \return 0 if all indices are valid, -EINVAL otherwise
> > > + */
> > > +int Thread::setThreadAffinity(const Span<const unsigned int> &cpus)
> > > +{
> > > +     const unsigned int numCpus = std::thread::hardware_concurrency();
> >
> > https://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency
> > says:
> >
> >   Returns the number of concurrent threads supported by the
> >   implementation. The value should be considered only a hint.
> >
> >   [snip]
> >
> >   Return value
> >
> >   Number of concurrent threads supported. If the value is not well
> >   defined or not computable, returns 0.
> >
> > libcxx seems to have the following implementation:
> >
> > unsigned thread::hardware_concurrency() noexcept {
> > #if defined(_SC_NPROCESSORS_ONLN)
> >   long result = sysconf(_SC_NPROCESSORS_ONLN);
> >   // sysconf returns -1 if the name is invalid, the option does not exist or
> >   // does not have a definite limit.
> >   // if sysconf returns some other negative number, we have no idea
> >   // what is going on. Default to something safe.
> >   if (result < 0)
> >     return 0;
> >   return static_cast<unsigned>(result);
> > #elif defined(_LIBCPP_WIN32API)
> >   SYSTEM_INFO info;
> >   GetSystemInfo(&info);
> >   return info.dwNumberOfProcessors;
> > #else // defined(CTL_HW) && defined(HW_NCPU)
> >   // TODO: grovel through /proc or check cpuid on x86 and similar
> >   // instructions on other architectures.
> > #  if defined(_LIBCPP_WARNING)
> >   _LIBCPP_WARNING("hardware_concurrency not yet implemented")
> > #  else
> > #    warning hardware_concurrency not yet implemented
> > #  endif
> >   return 0; // Means not computable [thread.thread.static]
> > #endif // defined(CTL_HW) && defined(HW_NCPU)
> > }
> >
> > while libstdc++ uses
> >
> > #if defined(_GLIBCXX_USE_GET_NPROCS)
> > # include <sys/sysinfo.h>
> > # define _GLIBCXX_NPROCS get_nprocs()
> > #elif defined(_GLIBCXX_USE_PTHREADS_NUM_PROCESSORS_NP)
> > # define _GLIBCXX_NPROCS pthread_num_processors_np()
> > #elif defined(_GLIBCXX_USE_SYSCTL_HW_NCPU)
> > # include <stddef.h>
> > # include <sys/sysctl.h>
> > static inline int get_nprocs()
> > {
> >  int count;
> >  size_t size = sizeof(count);
> >  int mib[] = { CTL_HW, HW_NCPU };
> >  if (!sysctl(mib, 2, &count, &size, NULL, 0))
> >    return count;
> >  return 0;
> > }
> > # define _GLIBCXX_NPROCS get_nprocs()
> > #elif defined(_GLIBCXX_USE_SC_NPROCESSORS_ONLN)
> > # include <unistd.h>
> > # define _GLIBCXX_NPROCS sysconf(_SC_NPROCESSORS_ONLN)
> > #elif defined(_GLIBCXX_USE_SC_NPROC_ONLN)
> > # include <unistd.h>
> > # define _GLIBCXX_NPROCS sysconf(_SC_NPROC_ONLN)
> > #else
> > # define _GLIBCXX_NPROCS 0
> > #endif
> >
> >   unsigned int
> >   thread::hardware_concurrency() noexcept
> >   {
> >     int __n = _GLIBCXX_NPROCS;
> >     if (__n < 0)
> >       __n = 0;
> >     return __n;
> >   }
> >
> > get_nprocs() and sysconf(_SC_NPROCESSORS_ONLN) are identical (the latter
> > is a wrapper around the former). So it seems that, while the C++ API
> > doesn't guarantee that thread::hardware_concurrency() will provide an
> > accurate value, the implementations we have will do the right thing.
> > They're however a bit expensive, as they open a file in /sys, but that's
> > probably the best we can do.
> >
> > > +
> > > +     MutexLocker locker(data_->mutex_);
> > > +     data_->cpuset_ = cpu_set_t();
> > > +     CPU_ZERO(&data_->cpuset_.value());
> > > +
> > > +     for (const unsigned int &cpu : cpus) {
> > > +             if (cpu >= numCpus) {
> > > +                     LOG(Thread, Error) << "Ignore an invalid cpu index: " << cpu;
> >
> > It's not ignored anymore
> >
> >                         LOG(Thread, Error)
> >                                 << "Invalid CPU " << cpu << " for thread affinity";
> >
> 
> Right, updated.
> 
> > > +                     return -EINVAL;
> > > +             }
> > > +
> > > +             CPU_SET(cpu, &data_->cpuset_.value());
> > > +     }
> > > +
> > > +     if (data_->running_)
> > > +             setThreadAffinityInternal();
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +void Thread::setThreadAffinityInternal()
> > > +{
> > > +     if (!data_->cpuset_)
> > > +             return;
> > > +
> > > +     const cpu_set_t &cpuset = data_->cpuset_.value();
> > > +     pthread_setaffinity_np(thread_.native_handle(), sizeof(cpuset), &cpuset);
> >
> > With this implementation, it's not possible to set the thread's CPU
> > affinity before starting the thread. That's a limitation of the C++
> > std::thread API, it could be done if we used the native pthread API
> > instead. Is that a problem ? Could you actually point me to code that
> > uses Thread::setThreadAffinity() ?
> 
> There are two main usages in mtkisp7:
> 1. Setting pipeline handler's thread [1].

Heads up on that one, we probably want to handle this in the libcamera
core. Multiple pipeline handlers can run on the same platform, currently
sharing a single camera manager thread. If they all tried to set thread
affinity, that would be messy. I'd like to explore how we could
coordinate that in a cleaner way.

> 2. Setting IPA's two new threads [2].
> 
> The first one sets thread affinity to a thread that has already been
> started, while the second one set new threads.

Out of curiosity, have you measured the difference in performance ? Is
it a matter of pinning the threads to big cores to ensure they have
enough CPU time ? Or a matter of avoiding latency caused by migration ?
Or something else ?

> By "the native pthread API", you mean `pthread_attr_setaffinity_np`?
> This means that we need to implement libcamera::Thread with pthread,
> instead of std::thread?

Yes. It doesn't mean we have to do that, I'm only asking if you think
that would be a good idea, or overkill.

> [1]: https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/third_party/libcamera/mtkisp7/src/libcamera/pipeline/mtkisp7/mtkisp7.cpp;l=511
> [2]: https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/third_party/libcamera/mtkisp7/src/ipa/mtkisp7/mtkisp7.cpp;l=119
> 
> > Even without the ability to set the affinity before starting the thread,
> > we could set it from within the thread before calling the run()
> > function. This would avoid the main workload being run on incorrect
> > CPUs. Is that something that would be useful ?
> 
> You mean calling `setThreadAffinityInterna()l` in `startThread()`
> (before `run()`), instead of at the end of `start()`?

Yes. Again, I'm not saing it has to be done, just exploring options.

> > Regardless of which option we implement, Thread::setThreadAffinity()
> > should document what happens if it's called before calling
> > Thread::start().
> >
> > > +}
> > > +
> > >  /**
> > >   * \brief Check if the thread is running
> > >   *

-- 
Regards,

Laurent Pinchart


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