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-rw-r--r--Documentation/accounting/psi.txt119
1 files changed, 113 insertions, 6 deletions
diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
index b8ca28b60215..5cbe5659e3b7 100644
--- a/Documentation/accounting/psi.txt
+++ b/Documentation/accounting/psi.txt
@@ -56,12 +56,116 @@ situation from a state where some tasks are stalled but the CPU is
still doing productive work. As such, time spent in this subset of the
stall state is tracked separately and exported in the "full" averages.
-The ratios are tracked as recent trends over ten, sixty, and three
-hundred second windows, which gives insight into short term events as
-well as medium and long term trends. The total absolute stall time is
-tracked and exported as well, to allow detection of latency spikes
-which wouldn't necessarily make a dent in the time averages, or to
-average trends over custom time frames.
+The ratios (in %) are tracked as recent trends over ten, sixty, and
+three hundred second windows, which gives insight into short term events
+as well as medium and long term trends. The total absolute stall time
+(in us) is tracked and exported as well, to allow detection of latency
+spikes which wouldn't necessarily make a dent in the time averages,
+or to average trends over custom time frames.
+
+Monitoring for pressure thresholds
+==================================
+
+Users can register triggers and use poll() to be woken up when resource
+pressure exceeds certain thresholds.
+
+A trigger describes the maximum cumulative stall time over a specific
+time window, e.g. 100ms of total stall time within any 500ms window to
+generate a wakeup event.
+
+To register a trigger user has to open psi interface file under
+/proc/pressure/ representing the resource to be monitored and write the
+desired threshold and time window. The open file descriptor should be
+used to wait for trigger events using select(), poll() or epoll().
+The following format is used:
+
+<some|full> <stall amount in us> <time window in us>
+
+For example writing "some 150000 1000000" into /proc/pressure/memory
+would add 150ms threshold for partial memory stall measured within
+1sec time window. Writing "full 50000 1000000" into /proc/pressure/io
+would add 50ms threshold for full io stall measured within 1sec time window.
+
+Triggers can be set on more than one psi metric and more than one trigger
+for the same psi metric can be specified. However for each trigger a separate
+file descriptor is required to be able to poll it separately from others,
+therefore for each trigger a separate open() syscall should be made even
+when opening the same psi interface file.
+
+Monitors activate only when system enters stall state for the monitored
+psi metric and deactivates upon exit from the stall state. While system is
+in the stall state psi signal growth is monitored at a rate of 10 times per
+tracking window.
+
+The kernel accepts window sizes ranging from 500ms to 10s, therefore min
+monitoring update interval is 50ms and max is 1s. Min limit is set to
+prevent overly frequent polling. Max limit is chosen as a high enough number
+after which monitors are most likely not needed and psi averages can be used
+instead.
+
+When activated, psi monitor stays active for at least the duration of one
+tracking window to avoid repeated activations/deactivations when system is
+bouncing in and out of the stall state.
+
+Notifications to the userspace are rate-limited to one per tracking window.
+
+The trigger will de-register when the file descriptor used to define the
+trigger is closed.
+
+Userspace monitor usage example
+===============================
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <poll.h>
+#include <string.h>
+#include <unistd.h>
+
+/*
+ * Monitor memory partial stall with 1s tracking window size
+ * and 150ms threshold.
+ */
+int main() {
+ const char trig[] = "some 150000 1000000";
+ struct pollfd fds;
+ int n;
+
+ fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK);
+ if (fds.fd < 0) {
+ printf("/proc/pressure/memory open error: %s\n",
+ strerror(errno));
+ return 1;
+ }
+ fds.events = POLLPRI;
+
+ if (write(fds.fd, trig, strlen(trig) + 1) < 0) {
+ printf("/proc/pressure/memory write error: %s\n",
+ strerror(errno));
+ return 1;
+ }
+
+ printf("waiting for events...\n");
+ while (1) {
+ n = poll(&fds, 1, -1);
+ if (n < 0) {
+ printf("poll error: %s\n", strerror(errno));
+ return 1;
+ }
+ if (fds.revents & POLLERR) {
+ printf("got POLLERR, event source is gone\n");
+ return 0;
+ }
+ if (fds.revents & POLLPRI) {
+ printf("event triggered!\n");
+ } else {
+ printf("unknown event received: 0x%x\n", fds.revents);
+ return 1;
+ }
+ }
+
+ return 0;
+}
Cgroup2 interface
=================
@@ -71,3 +175,6 @@ mounted, pressure stall information is also tracked for tasks grouped
into cgroups. Each subdirectory in the cgroupfs mountpoint contains
cpu.pressure, memory.pressure, and io.pressure files; the format is
the same as the /proc/pressure/ files.
+
+Per-cgroup psi monitors can be specified and used the same way as
+system-wide ones.