On Dec 14, 2017, at 3:00 PM, Krzysztof Opasiak [off-list ref] wrote:

Add rlimit-events calls to file descriptors management
code to allow tracing of FD usage.

This allows userspace process (monitor) to get notification when
other process (subject) uses given amount of file descriptors.

This can be used to for example asynchronously monitor number
of open FD's in system services instead of polling with
predefined interval.

I'm not involved in this area of code, but one optimization question:

+static unsigned int count_open_fds(struct fdtable *fdt)
+{
+	unsigned int maxfd = fdt->max_fds;
+	unsigned int maxbit = maxfd / BITS_PER_LONG;
+	unsigned int count = 0;
+	int i;
+
+	i = find_next_zero_bit(fdt->full_fds_bits, maxbit, 0);
+	/* If there is no free fds */
+	if (i > maxbit)
+		return maxfd;
+#if BITS_PER_LONG == 32
+#define HWEIGHT_LONG hweight32
+#else
+#define HWEIGHT_LONG hweight64
+#endif
+
+	count += i * BITS_PER_LONG;
+	for (; i < maxbit; ++i)
+		count += HWEIGHT_LONG(fdt->open_fds[i]);
+
+#undef HWEIGHT_LONG
+	return count;
+}

Since find_next_zero_bit() needs to process all of the words anyway
as well as lots of extra operations that add overhead, it looks more
efficient to just compute HWEIGHT_LONG(open_fds[]) for the whole array.

Cheers, Andreas