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health: Start probe timeout after exec starts

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Starting an exec can take a significant amount of time while under heavy
container operation load. In extreme cases the time to start the process
can take upwards of a second, which is a significant fraction of the
default health probe timeout (30s). With a shorter timeout, the exec
start delay could make the difference between a successful probe and a
probe timeout! Mitigate the impact of excessive exec start latencies by
only starting the probe timeout timer after the exec'ed process has
started.

Add a metric to sample the latency of starting health-check exec probes.

Signed-off-by: Cory Snider <csnider@mirantis.com>
This commit is contained in:
Cory Snider 2022-04-12 17:57:23 -04:00
parent 3fd01fc3c6
commit bdc6473d2d
2 changed files with 48 additions and 18 deletions
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61
daemon/health.go
View file

@ -62,6 +62,7 @@ type cmdProbe struct {
// exec the healthcheck command in the container.
// Returns the exit code and probe output (if any)
func (p *cmdProbe) run(ctx context.Context, d *Daemon, cntr *container.Container) (*types.HealthcheckResult, error) {
startTime := time.Now()
cmdSlice := strslice.StrSlice(cntr.Config.Healthcheck.Test)[1:]
if p.shell {
cmdSlice = append(getShell(cntr), cmdSlice...)
@ -93,10 +94,52 @@ func (p *cmdProbe) run(ctx context.Context, d *Daemon, cntr *container.Container
d.LogContainerEventWithAttributes(cntr, "exec_create: "+execConfig.Entrypoint+" "+strings.Join(execConfig.Args, " "), attributes)
output := &limitedBuffer{}
err = d.ContainerExecStart(ctx, execConfig.ID, nil, output, output)
probeCtx, cancelProbe := context.WithCancel(ctx)
defer cancelProbe()
execErr := make(chan error, 1)
go func() { execErr <- d.ContainerExecStart(probeCtx, execConfig.ID, nil, output, output) }()
// Starting an exec can take a significant amount of time: on the order
// of 1s in extreme cases. The time it takes dockerd and containerd to
// start the exec is time that the probe process is not running, and so
// should not count towards the health check's timeout. Apply a separate
// timeout to abort if the exec request is wedged.
tm := time.NewTimer(30 * time.Second)
defer tm.Stop()
select {
case <-tm.C:
return nil, fmt.Errorf("timed out starting health check for container %s", cntr.ID)
case err := <-execErr:
if err != nil {
return nil, err
}
case <-execConfig.Started:
healthCheckStartDuration.UpdateSince(startTime)
}
if !tm.Stop() {
<-tm.C
}
probeTimeout := timeoutWithDefault(cntr.Config.Healthcheck.Timeout, defaultProbeTimeout)
tm.Reset(probeTimeout)
select {
case <-tm.C:
cancelProbe()
logrus.WithContext(ctx).Debugf("Health check for container %s taking too long", cntr.ID)
// Wait for probe to exit (it might take a while to respond to the TERM
// signal and we don't want dying probes to pile up).
<-execErr
return &types.HealthcheckResult{
ExitCode: -1,
Output: fmt.Sprintf("Health check exceeded timeout (%v)", probeTimeout),
End: time.Now(),
}, nil
case err := <-execErr:
if err != nil {
return nil, err
}
}
info, err := d.getExecConfig(execConfig.ID)
if err != nil {
return nil, err
@ -184,7 +227,6 @@ func handleProbeResult(d *Daemon, c *container.Container, result *types.Healthch
// Run the container's monitoring thread until notified via "stop".
// There is never more than one monitor thread running per container at a time.
func monitor(d *Daemon, c *container.Container, stop chan struct{}, probe probe) {
probeTimeout := timeoutWithDefault(c.Config.Healthcheck.Timeout, defaultProbeTimeout)
probeInterval := timeoutWithDefault(c.Config.Healthcheck.Interval, defaultProbeInterval)
intervalTimer := time.NewTimer(probeInterval)
@ -200,7 +242,7 @@ func monitor(d *Daemon, c *container.Container, stop chan struct{}, probe probe)
case <-intervalTimer.C:
logrus.Debugf("Running health check for container %s ...", c.ID)
startTime := time.Now()
ctx, cancelProbe := context.WithTimeout(context.Background(), probeTimeout)
ctx, cancelProbe := context.WithCancel(context.Background())
results := make(chan *types.HealthcheckResult, 1)
go func() {
healthChecksCounter.Inc()
@ -231,20 +273,7 @@ func monitor(d *Daemon, c *container.Container, stop chan struct{}, probe probe)
return
case result := <-results:
handleProbeResult(d, c, result, stop)
// Stop timeout
cancelProbe()
case <-ctx.Done():
logrus.Debugf("Health check for container %s taking too long", c.ID)
handleProbeResult(d, c, &types.HealthcheckResult{
ExitCode: -1,
Output: fmt.Sprintf("Health check exceeded timeout (%v)", probeTimeout),
Start: startTime,
End: time.Now(),
}, stop)
cancelProbe()
// Wait for probe to exit (it might take a while to respond to the TERM
// signal and we don't want dying probes to pile up).
<-results
}
}
}

1
daemon/metrics.go
View file

@ -37,6 +37,7 @@ var (
healthChecksCounter = metricsNS.NewCounter("health_checks", "The total number of health checks")
healthChecksFailedCounter = metricsNS.NewCounter("health_checks_failed", "The total number of failed health checks")
healthCheckStartDuration = metricsNS.NewTimer("health_check_start_duration", "The number of seconds it takes to prepare to run health checks")
stateCtr = newStateCounter(metricsNS, metricsNS.NewDesc("container_states", "The count of containers in various states", metrics.Unit("containers"), "state"))
)