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Batch threads waiting for the monitor's attention.

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Instead of doing waitpid() and processing one thread at a time, gather all waiting threads and then process them.

This avoids starving older threads when newer threads raise a lot of events.

PiperOrigin-RevId: 466366533
Change-Id: I81a878f038feac86407a8e961ecba181004f0f8a
This commit is contained in:
Sandboxed API Team 2022-08-09 08:27:28 -07:00 committed by Copybara-Service
parent 26b2519aed
commit deb3c8e77b
1 changed files with 66 additions and 16 deletions
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82
sandboxed_api/sandbox2/monitor.cc
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@ -36,11 +36,13 @@
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <memory>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <glog/logging.h>
#include "absl/cleanup/cleanup.h"
@ -88,6 +90,67 @@ namespace sandbox2 {
namespace {
// Since waitpid() is biased towards newer threads, we run the risk of starving
// older threads if the newer ones raise a lot of events.
// To avoid it, we use this class to gather all the waiting threads and then
// return them one at a time on each call to Wait().
// In this way, everyone gets their chance.
class PidWaiter {
public:
// Constructs a PidWaiter where the given priority_pid is checked first.
explicit PidWaiter(pid_t priority_pid) : priority_pid_(priority_pid) {}
// Returns the PID of a thread that needs attention, populating 'status' with
// the status returned by the waitpid() call. It returns 0 if no threads
// require attention at the moment, or -1 if there was an error, in which case
// the error value can be found in 'errno'.
int Wait(int* status) {
if (statuses_.empty() && last_errno_ == 0) {
RefillStatuses();
}
if (statuses_.empty()) {
if (last_errno_ == 0) return 0;
errno = last_errno_;
last_errno_ = 0;
return -1;
}
const auto& entry = statuses_.front();
pid_t pid = entry.first;
*status = entry.second;
statuses_.pop_front();
return pid;
}
private:
void RefillStatuses() {
statuses_.clear();
last_errno_ = 0;
pid_t pid = priority_pid_;
int status;
while (true) {
// It should be a non-blocking operation (hence WNOHANG), so this function
// returns quickly if there are no events to be processed.
pid_t ret =
waitpid(pid, &status, __WNOTHREAD | __WALL | WUNTRACED | WNOHANG);
if (ret > 0) {
statuses_.emplace_back(ret, status);
} else if (ret < 0) {
last_errno_ = errno;
break;
} else if (pid == -1) {
break;
}
pid = -1;
}
}
pid_t priority_pid_;
std::deque<std::pair<pid_t, int>> statuses_ = {};
int last_errno_ = 0;
};
// We could use the ProcMapsIterator, however we want the full file content.
std::string ReadProcMaps(pid_t pid) {
std::ifstream input(absl::StrCat("/proc/", pid, "/maps"),
@ -430,6 +493,7 @@ bool Monitor::InterruptSandboxee() {
void Monitor::MainLoop(sigset_t* sset) {
bool sandboxee_exited = false;
PidWaiter pid_waiter(pid_);
int status;
// All possible still running children of main process, will be killed due to
// PTRACE_O_EXITKILL ptrace() flag.
@ -467,15 +531,7 @@ void Monitor::MainLoop(sigset_t* sset) {
}
}
// It should be a non-blocking operation (hence WNOHANG), so this function
// returns quickly if there are no events to be processed.
// Prioritize main pid to avoid resource starvation
pid_t ret =
waitpid(pid_, &status, __WNOTHREAD | __WALL | WUNTRACED | WNOHANG);
if (ret == 0) {
ret = waitpid(-1, &status, __WNOTHREAD | __WALL | WUNTRACED | WNOHANG);
}
pid_t ret = pid_waiter.Wait(&status);
if (ret == 0) {
constexpr timespec ts = {kWakeUpPeriodSec, kWakeUpPeriodNSec};
int signo = sigtimedwait(sset, nullptr, &ts);
@ -552,13 +608,7 @@ void Monitor::MainLoop(sigset_t* sset) {
LOG(INFO) << "Waiting for sandboxee exit timed out";
break;
}
pid_t ret =
waitpid(pid_, &status, __WNOTHREAD | __WALL | WUNTRACED | WNOHANG);
if (ret == 0) {
// Sometimes PTRACE_EVENT_EXIT needs to be handled for each child thread
// in order to observe main thread exit
ret = waitpid(-1, &status, __WNOTHREAD | __WALL | WUNTRACED | WNOHANG);
}
pid_t ret = pid_waiter.Wait(&status);
if (ret == -1) {
PLOG(ERROR) << "waitpid() failed";
break;