[PATCH] fix f_version optimization for get_tgid_list
From: Manfred Spraul (manfred_at_colorfullife.com)
Date: 08/31/04
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Date: Tue, 31 Aug 2004 22:26:35 +0200 To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Hi,
the kernel contains an optimization that skips the linked list walk in
get_tgid_list for the common case of sequential accesses.
Unfortunately the optimization is buggy (missing NULL pointer check for
the result of find_task_by_pid) and broken (actually - broken twice: the
tgid value that is stored in f_version is always 0 because tgid is
overwritten when the string is created and additionally the common case
is not filldir < 0, it's running out of nr_tgids).
The attached patch fixes these bugs.
Roger: could you run your 100k processes test against a kernel with this
fix applied? I'm just interested how much it helps. One obvious result
are fewer getdents64 syscalls: instead of returning 20 entries (480
bytes) the new code fills the user space buffer completely (42
entries/syscall).
Andrew, could you add the patch to your -mm tree?
Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
--- 2.6/fs/proc/base.c 2004-08-20 19:59:19.000000000 +0200
+++ build-2.6/fs/proc/base.c 2004-08-31 21:42:28.000000000 +0200
@@ -1689,7 +1689,7 @@ static int get_tgid_list(int index, unsi
p = NULL;
if (version) {
p = find_task_by_pid(version);
- if (!thread_group_leader(p))
+ if (p && !thread_group_leader(p))
p = NULL;
}
@@ -1752,6 +1752,7 @@ int proc_pid_readdir(struct file * filp,
char buf[PROC_NUMBUF];
unsigned int nr = filp->f_pos - FIRST_PROCESS_ENTRY;
unsigned int nr_tgids, i;
+ int next_tgid;
if (!nr) {
ino_t ino = fake_ino(0,PROC_TGID_INO);
@@ -1761,26 +1762,45 @@ int proc_pid_readdir(struct file * filp,
nr++;
}
- /*
- * f_version caches the last tgid which was returned from readdir
+ /* f_version caches the tgid value that the last readdir call couldn't
+ * return. lseek aka telldir automagically resets f_version to 0.
*/
- nr_tgids = get_tgid_list(nr, filp->f_version, tgid_array);
-
- for (i = 0; i < nr_tgids; i++) {
- int tgid = tgid_array[i];
- ino_t ino = fake_ino(tgid,PROC_TGID_INO);
- unsigned long j = PROC_NUMBUF;
-
- do
- buf[--j] = '0' + (tgid % 10);
- while ((tgid /= 10) != 0);
-
- if (filldir(dirent, buf+j, PROC_NUMBUF-j, filp->f_pos, ino, DT_DIR) < 0) {
- filp->f_version = tgid;
+ next_tgid = filp->f_version;
+ filp->f_version = 0;
+ for (;;) {
+ nr_tgids = get_tgid_list(nr, next_tgid, tgid_array);
+ if (!nr_tgids) {
+ /* no more entries ! */
break;
}
- filp->f_pos++;
+ next_tgid = 0;
+
+ /* do not use the last found pid, reserve it for next_tgid */
+ if (nr_tgids == PROC_MAXPIDS) {
+ nr_tgids--;
+ next_tgid = tgid_array[nr_tgids];
+ }
+
+ for (i=0;i<nr_tgids;i++) {
+ int tgid = tgid_array[i];
+ ino_t ino = fake_ino(tgid,PROC_TGID_INO);
+ unsigned long j = PROC_NUMBUF;
+
+ do
+ buf[--j] = '0' + (tgid % 10);
+ while ((tgid /= 10) != 0);
+
+ if (filldir(dirent, buf+j, PROC_NUMBUF-j, filp->f_pos, ino, DT_DIR) < 0) {
+ /* returning this tgid failed, save it as the first
+ * pid for the next readir call */
+ filp->f_version = tgid_array[i];
+ goto out;
+ }
+ filp->f_pos++;
+ nr++;
+ }
}
+out:
return 0;
}
-
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