Perf.h

Go to the documentation of this file.

//===-- Perf.h --------------------------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
/// \file
/// This file contains a thin wrapper of the perf_event_open API
/// and classes to handle the destruction of file descriptors
/// and mmap pointers.
///
//===----------------------------------------------------------------------===//
 
#ifndef LLDB_SOURCE_PLUGINS_PROCESS_LINUX_PERF_H
#define LLDB_SOURCE_PLUGINS_PROCESS_LINUX_PERF_H
 
#include "lldb/Utility/TraceIntelPTGDBRemotePackets.h"
#include "lldb/lldb-types.h"
#include "llvm/Support/Error.h"
#include <chrono>
#include <cstdint>
#include <linux/perf_event.h>
 
namespace lldb_private {
namespace process_linux {
namespace resource_handle {
 
/// Custom deleter for the pointer returned by \a mmap.
///
/// This functor type is provided to \a unique_ptr to properly
/// unmap the region at destruction time.
class MmapDeleter {
public:
  /// Construct new \a MmapDeleter.
  ///
  /// \param[in] bytes
  ///   Size of the mmap'ed region in bytes.
  MmapDeleter(size_t bytes = 0) : m_bytes(bytes) {}
 
  /// Unmap the mmap'ed region.
  ///
  /// If \a m_bytes==0 or \a ptr==nullptr, nothing is unmmapped.
  ///
  /// \param[in] ptr
  ///   pointer to the region to be unmmapped.
  void operator()(void *ptr);
 
private:
  /// Size of the mmap'ed region, in bytes, to be unmapped.
  size_t m_bytes;
};
 
/// Custom deleter for a file descriptor.
///
/// This functor type is provided to \a unique_ptr to properly release
/// the resources associated with the file descriptor at destruction time.
class FileDescriptorDeleter {
public:
  /// Close and free the memory associated with the file descriptor pointer.
  ///
  /// Effectively a no-op if \a ptr==nullptr or \a*ptr==-1.
  ///
  /// \param[in] ptr
  ///   Pointer to the file descriptor.
  void operator()(long *ptr);
};
 
using FileDescriptorUP =
    std::unique_ptr<long, resource_handle::FileDescriptorDeleter>;
using MmapUP = std::unique_ptr<void, resource_handle::MmapDeleter>;
 
} // namespace resource_handle
 
/// Thin wrapper of the perf_event_open API.
///
/// Exposes the metadata page and data and aux buffers of a perf event.
/// Handles the management of the event's file descriptor and mmap'ed
/// regions.
class PerfEvent {
public:
  /// Create a new performance monitoring event via the perf_event_open syscall.
  ///
  /// The parameters are directly forwarded to a perf_event_open syscall,
  /// for additional information on the parameters visit
  /// https://man7.org/linux/man-pages/man2/perf_event_open.2.html.
  ///
  /// \param[in] attr
  ///     Configuration information for the event.
  ///
  /// \param[in] pid
  ///     The process or thread to be monitored by the event. If \b None, then
  ///     all processes and threads are monitored.
  ///
  /// \param[in] cpu
  ///     The cpu to be monitored by the event. If \b None, then all cpus are
  ///     monitored.
  ///
  /// \param[in] group_fd
  ///     File descriptor of the group leader. If \b None, then this perf_event
  ///     doesn't belong to a preexisting group.
  ///
  /// \param[in] flags
  ///     Bitmask of additional configuration flags.
  ///
  /// \return
  ///     If the perf_event_open syscall was successful, a minimal \a PerfEvent
  ///     instance, or an \a llvm::Error otherwise.
  static llvm::Expected<PerfEvent> Init(perf_event_attr &attr,
                                        std::optional<lldb::pid_t> pid,
                                        std::optional<lldb::cpu_id_t> cpu,
                                        std::optional<long> group_fd,
                                        unsigned long flags);
 
  /// Create a new performance monitoring event via the perf_event_open syscall
  /// with "default" values for the cpu, group_fd and flags arguments.
  ///
  /// Convenience method to be used when the perf event requires minimal
  /// configuration. It handles the default values of all other arguments.
  ///
  /// \param[in] attr
  ///     Configuration information for the event.
  ///
  /// \param[in] pid
  ///     The process or thread to be monitored by the event. If \b
  ///     std::nullopt, then all threads and processes are monitored.
  static llvm::Expected<PerfEvent>
  Init(perf_event_attr &attr, std::optional<lldb::pid_t> pid,
       std::optional<lldb::cpu_id_t> core = std::nullopt);
 
  /// Mmap the metadata page and the data and aux buffers of the perf event and
  /// expose them through \a PerfEvent::GetMetadataPage() , \a
  /// PerfEvent::GetDataBuffer() and \a PerfEvent::GetAuxBuffer().
  ///
  /// This uses mmap underneath, which means that the number of pages mmap'ed
  /// must be less than the actual data available by the kernel. The metadata
  /// page is always mmap'ed.
  ///
  /// Mmap is needed because the underlying data might be changed by the kernel
  /// dynamically.
  ///
  /// \param[in] num_data_pages
  ///     Number of pages in the data buffer to mmap, must be a power of 2.
  ///     A value of 0 is useful for "dummy" events that only want to access
  ///     the metadata, \a perf_event_mmap_page, or the aux buffer.
  ///
  /// \param[in] num_aux_pages
  ///     Number of pages in the aux buffer to mmap, must be a power of 2.
  ///     A value of 0 effectively is a no-op and no data is mmap'ed for this
  ///     buffer.
  ///
  /// \param[in] data_buffer_write
  ///     Whether to mmap the data buffer with WRITE permissions. This changes
  ///     the behavior of how the kernel writes to the data buffer.
  ///
  /// \return
  ///   \a llvm::Error::success if the mmap operations succeeded,
  ///   or an \a llvm::Error otherwise.
  llvm::Error MmapMetadataAndBuffers(size_t num_data_pages,
                                     size_t num_aux_pages,
                                     bool data_buffer_write);
 
  /// Get the file descriptor associated with the perf event.
  long GetFd() const;
 
  /// Get the metadata page from the data section's mmap buffer.
  ///
  /// The metadata page is always mmap'ed, even when \a num_data_pages is 0.
  ///
  /// This should be called only after \a PerfEvent::MmapMetadataAndBuffers,
  /// otherwise a failure might happen.
  ///
  /// \return
  ///   The data section's \a perf_event_mmap_page.
  perf_event_mmap_page &GetMetadataPage() const;
 
  /// Get the data buffer from the data section's mmap buffer.
  ///
  /// The data buffer is the region of the data section's mmap buffer where
  /// perf sample data is located.
  ///
  /// This should be called only after \a PerfEvent::MmapMetadataAndBuffers,
  /// otherwise a failure might happen.
  ///
  /// \return
  ///   \a ArrayRef<uint8_t> extending \a data_size bytes from \a data_offset.
  llvm::ArrayRef<uint8_t> GetDataBuffer() const;
 
  /// Get the AUX buffer.
  ///
  /// AUX buffer is a region for high-bandwidth data streams
  /// such as IntelPT. This is separate from the metadata and data buffer.
  ///
  /// This should be called only after \a PerfEvent::MmapMetadataAndBuffers,
  /// otherwise a failure might happen.
  ///
  /// \return
  ///   \a ArrayRef<uint8_t> extending \a aux_size bytes from \a aux_offset.
  llvm::ArrayRef<uint8_t> GetAuxBuffer() const;
 
  /// Read the aux buffer managed by this perf event assuming it was configured
  /// with PROT_READ permissions only, which indicates that the buffer is
  /// automatically wrapped and overwritten by the kernel or hardware. To ensure
  /// that the data is up-to-date and is not corrupted by read-write race
  /// conditions, the underlying perf_event is paused during read, and later
  /// it's returned to its initial state. The returned data will be linear, i.e.
  /// it will fix the circular wrapping the might exist in the buffer.
  ///
  /// \return
  ///     A vector with the requested binary data.
  llvm::Expected<std::vector<uint8_t>> GetReadOnlyAuxBuffer();
 
  /// Read the data buffer managed by this perf even assuming it was configured
  /// with PROT_READ permissions only, which indicates that the buffer is
  /// automatically wrapped and overwritten by the kernel or hardware. To ensure
  /// that the data is up-to-date and is not corrupted by read-write race
  /// conditions, the underlying perf_event is paused during read, and later
  /// it's returned to its initial state. The returned data will be linear, i.e.
  /// it will fix the circular wrapping the might exist int he buffer.
  ///
  /// \return
  ///     A vector with the requested binary data.
  llvm::Expected<std::vector<uint8_t>> GetReadOnlyDataBuffer();
 
  /// Use the ioctl API to disable the perf event and all the events in its
  /// group. This doesn't terminate the perf event.
  ///
  /// This is no-op if the perf event is already disabled.
  ///
  /// \return
  ///   An Error if the perf event couldn't be disabled.
  llvm::Error DisableWithIoctl();
 
  /// Use the ioctl API to enable the perf event and all the events in its
  /// group.
  ///
  /// This is no-op if the perf event is already enabled.
  ///
  /// \return
  ///   An Error if the perf event couldn't be enabled.
  llvm::Error EnableWithIoctl();
 
  /// \return
  ///   The size in bytes of the section of the data buffer that has effective
  ///   data.
  size_t GetEffectiveDataBufferSize() const;
 
  /// \return
  ///   \b true if and only the perf event is enabled and collecting.
  bool IsEnabled() const;
 
private:
  /// Create new \a PerfEvent.
  ///
  /// \param[in] fd
  ///   File descriptor of the perf event.
  ///
  /// \param[in] enabled
  ///   Initial collection state configured for this perf_event.
  PerfEvent(long fd, bool enabled)
      : m_fd(new long(fd), resource_handle::FileDescriptorDeleter()),
        m_enabled(enabled) {}
 
  /// Wrapper for \a mmap to provide custom error messages.
  ///
  /// The parameters are directly forwarded to a \a mmap syscall,
  /// for information on the parameters visit
  /// https://man7.org/linux/man-pages/man2/mmap.2.html.
  ///
  /// The value of \a GetFd() is passed as the \a fd argument to \a mmap.
  llvm::Expected<resource_handle::MmapUP> DoMmap(void *addr, size_t length,
                                                 int prot, int flags,
                                                 long int offset,
                                                 llvm::StringRef buffer_name);
 
  /// Mmap the data buffer of the perf event.
  ///
  /// \param[in] num_data_pages
  ///     Number of pages in the data buffer to mmap, must be a power of 2.
  ///     A value of 0 is useful for "dummy" events that only want to access
  ///     the metadata, \a perf_event_mmap_page, or the aux buffer.
  ///
  /// \param[in] data_buffer_write
  ///     Whether to mmap the data buffer with WRITE permissions. This changes
  ///     the behavior of how the kernel writes to the data buffer.
  llvm::Error MmapMetadataAndDataBuffer(size_t num_data_pages,
                                        bool data_buffer_write);
 
  /// Mmap the aux buffer of the perf event.
  ///
  /// \param[in] num_aux_pages
  ///   Number of pages in the aux buffer to mmap, must be a power of 2.
  ///   A value of 0 effectively is a no-op and no data is mmap'ed for this
  ///   buffer.
  llvm::Error MmapAuxBuffer(size_t num_aux_pages);
 
  /// The file descriptor representing the perf event.
  resource_handle::FileDescriptorUP m_fd;
  /// Metadata page and data section where perf samples are stored.
  resource_handle::MmapUP m_metadata_data_base;
  /// AUX buffer is a separate region for high-bandwidth data streams
  /// such as IntelPT.
  resource_handle::MmapUP m_aux_base;
  /// The state of the underlying perf_event.
  bool m_enabled;
};
 
/// Create a perf event that tracks context switches on a cpu.
///
/// \param[in] cpu_id
///   The core to trace.
///
/// \param[in] parent_perf_event
///   An optional perf event that will be grouped with the
///   new perf event.
llvm::Expected<PerfEvent>
CreateContextSwitchTracePerfEvent(lldb::cpu_id_t cpu_id,
                                  const PerfEvent *parent_perf_event = nullptr);
 
/// Load \a PerfTscConversionParameters from \a perf_event_mmap_page, if
/// available.
llvm::Expected<LinuxPerfZeroTscConversion> LoadPerfTscConversionParameters();
 
} // namespace process_linux
} // namespace lldb_private
 
#endif // LLDB_SOURCE_PLUGINS_PROCESS_LINUX_PERF_H