[Xen-devel] [PATCH v3] tools/xenconsoled: Increase file descriptor limit

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

XenServer's VM density testing uncovered a regression when moving from
sysvinit to systemd where the file descriptor limit dropped from 4096 to
1024. (XenServer had previously inserted a ulimit statement into its
initscripts.)

One solution is to use LimitNOFILE=4096 in xenconsoled.service to match the
lost ulimit, but that is only a stopgap solution.

As Xenconsoled genuinely needs a large number of file descriptors if a large
number of domains are running, and is well behaved with its descriptors,
attempt to up the limit to the system maximum.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>

---
v3:
 * Hide Linux specific bits in #ifdef __linux__
v2:
 * Always increase soft limit to hard limit
 * Correct commment regarding number of file descriptors
 * long -> unsigned long as that appears to be the underlying type of an rlim_t
---
 tools/console/daemon/main.c |   47 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/tools/console/daemon/main.c b/tools/console/daemon/main.c
index 92d2fc4..20ffe6b 100644
--- a/tools/console/daemon/main.c
+++ b/tools/console/daemon/main.c
@@ -26,6 +26,7 @@
 #include <string.h>
 #include <signal.h>
 #include <sys/types.h>
+#include <sys/resource.h>
 
 #include "xenctrl.h"
 
@@ -55,6 +56,50 @@ static void version(char *name)
        printf("Xen Console Daemon 3.0\n");
 }
 
+/*
+ * Xenconsoled requires two file descriptors for each PV console (pty and log
+ * file), which can easily exceed the default of 1024 if many guests are
+ * running.  Try to set the fd limit to the system maximum, falling back to a
+ * default of 4096.
+ */
+static void increase_fd_limit(void)
+{
+       FILE *fs_nr_open;
+       struct rlimit lim;
+       unsigned long nr_open = 4096;
+
+       if (getrlimit(RLIMIT_NOFILE, &lim) < 0)
+               return;
+
+       /* Increase the soft limit to the current hard limit. */
+       if (lim.rlim_cur < lim.rlim_max) {
+               lim.rlim_cur = lim.rlim_max;
+               if (setrlimit(RLIMIT_NOFILE, &lim) < 0)
+                       return;
+       }
+
+#ifdef __linux__
+       /* Attempt to locate the system maximum. */
+       fs_nr_open = fopen("/proc/sys/fs/nr_open", "r");
+       if (fs_nr_open) {
+               unsigned long nr;
+
+               if (fscanf(fs_nr_open, "%lu", &nr) == 1)
+                       nr_open = nr;
+               fclose(fs_nr_open);
+       }
+#endif
+
+       /*
+        * In the likely case that we are a root process with
+        * CAP_SYS_RESOURCE, attempt to up our hard limit.
+        */
+       if (nr_open > lim.rlim_max) {
+               lim.rlim_cur = lim.rlim_max = nr_open;
+               setrlimit(RLIMIT_NOFILE, &lim);
+       }
+}
+
 int main(int argc, char **argv)
 {
        const char *sopts = "hVvit:o:";
@@ -154,6 +199,8 @@ int main(int argc, char **argv)
        openlog("xenconsoled", syslog_option, LOG_DAEMON);
        setlogmask(syslog_mask);
 
+       increase_fd_limit();
+
        if (!is_interactive) {
                daemonize(pidfile ? pidfile : "/var/run/xenconsoled.pid");
        }
-- 
1.7.10.4


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