Add algorithm to programmatically enable/disable perf

GaudiProfiling/src/component/perf/PerfProfile.cpp

+/***********************************************************************************\
+* (c) Copyright 1998-2023 CERN for the benefit of the LHCb and ATLAS collaborations *
+*                                                                                   *
+* This software is distributed under the terms of the Apache version 2 licence,     *
+* copied verbatim in the file "LICENSE".                                            *
+*                                                                                   *
+* In applying this licence, CERN does not waive the privileges and immunities       *
+* granted to it by virtue of its status as an Intergovernmental Organization        *
+* or submit itself to any jurisdiction.                                             *
+\***********************************************************************************/
+#include "GaudiAlg/Consumer.h"
+#include <fcntl.h>
+#include <string_view>
+#include <sys/stat.h>
+
+/** Algorithm to enable/disable profiling with Linux perf at given events.
+ *
+ *  Needs at least perf 5.9. To control perf record, start it as
+ *
+ *    perf record -D -1 --control fifo:GaudiPerfProfile.fifo ... gaudirun.py ...
+ *
+ *  The path to the control fifo (GaudiPerfProfile.fifo) is configurable
+ *  with the FIFOPath property. The fifo must be created before running perf.
+ *
+ */
+struct PerfProfile final : Gaudi::Functional::Consumer<void()> {
+  using Consumer::Consumer;
+
+  StatusCode initialize() override {
+    return Consumer::initialize().andThen( [this]() {
+      m_fifo = ::open( m_fifoPath.value().c_str(), O_WRONLY | O_NONBLOCK );
+      if ( m_fifo == -1 ) {
+        fatal() << "open(\"" << m_fifoPath.value() << "\"): " << ::strerror( errno ) << endmsg;
+        return StatusCode::FAILURE;
+      }
+      return StatusCode::SUCCESS;
+    } );
+  }
+
+  StatusCode finalize() override {
+    if ( m_fifo != -1 ) { ::close( m_fifo ); }
+    return Consumer::finalize();
+  }
+
+  void operator()() const override {
+    // We could use EventContext::evt(), however it is not always valid, as is the case with EvtSel="NONE". Instead, use
+    // an atomic counter.
+    auto eventNumber = m_eventNumber++;
+    if ( eventNumber == m_nStartFromEvent.value() ) {
+      warning() << "Starting perf profile at event " << eventNumber << endmsg;
+      fifo_write( "enable\n" );
+    }
+
+    if ( m_nStopAtEvent > 0 && eventNumber == m_nStopAtEvent.value() ) {
+      warning() << "Stopping perf profile at event " << eventNumber << endmsg;
+      fifo_write( "disable\n" );
+    }
+  }
+
+private:
+  void fifo_write( std::string_view s ) const {
+    if ( ::write( m_fifo, s.data(), s.size() ) < ssize_t( s.size() ) ) {
+      error() << "Write of \"" << s << "\" to FIFO failed: " << ::strerror( errno ) << endmsg;
+    }
+  }
+
+  mutable std::atomic<long unsigned> m_eventNumber = 0;
+  int                                m_fifo        = -1;
+
+  Gaudi::Property<std::string>   m_fifoPath{ this, "FIFOPath", "GaudiPerfProfile.fifo", "Path to perf control FIFO." };
+  Gaudi::Property<unsigned long> m_nStartFromEvent{ this, "StartFromEventN", 1,
+                                                    "After what event we start profiling." };
+  Gaudi::Property<unsigned long> m_nStopAtEvent{
+      this, "StopAtEventN", 0,
+      "After what event we stop profiling. If 0 than we also profile finalization stage. Default = 0." };
+};
+
+DECLARE_COMPONENT( PerfProfile )