O15int for interactivity

This patch addresses the problem of tasks that preempt their children when they're forking, wasting cpu cycles until they get demoted to a priority where they no longer preempt their child. Although this has been said to be a design flaw in the applications, it can cause sustained periods of starvation due to this coding problem, and the more I looked, the more examples I found of this happening.

Tasks now cannot preempt their own children. This speeds up the startup of child applications (eg pgp signed email).

This change has allowed tasks to stay at higher priority for much longer so the sleep avg decay of high credit tasks has been changed to match the rate of rise during periods of sleep (which I wanted to do originally but was limited by the first problem). This makes for much more sustained interactivity at extreme loads, and much less X jerkiness.

Con

Patch against 2.6.0-test3-mm1:

@@ -620,8 +620,13 @@ repeat_lock_task:
 				__activate_task(p, rq);
 			else {
 				activate_task(p, rq);
-				if (TASK_PREEMPTS_CURR(p, rq))
-					resched_task(rq->curr);
+				/*
+				 * Parents are not allowed to preempt their
+				 * children
+				 */
+				if (TASK_PREEMPTS_CURR(p, rq) &&
+					p != rq->curr->parent)
+						resched_task(rq->curr);
 			}
 			success = 1;
 		}

@@ -1124,7 +1129,7 @@ static inline void pull_task(runqueue_t 
 	 * Note that idle threads have a prio of MAX_PRIO, for this test
 	 * to be always true for them.
 	 */
-	if (TASK_PREEMPTS_CURR(p, this_rq))
+	if (TASK_PREEMPTS_CURR(p, this_rq) && p != this_rq->curr->parent)
 		set_need_resched();
 }
 

@@ -1493,9 +1498,8 @@ need_resched:
 	 * priority bonus
 	 */
 	if (HIGH_CREDIT(prev))
-		run_time /= (MAX_BONUS + 1 -
-			(NS_TO_JIFFIES(prev->sleep_avg) * MAX_BONUS /
-			MAX_SLEEP_AVG));
+		run_time /= ((NS_TO_JIFFIES(prev->sleep_avg) * MAX_BONUS /
+				MAX_SLEEP_AVG) ? : 1);
 
 	spin_lock_irq(&rq->lock);
 

I'd appreciate patches against the previous version, in this case O14.1, as well as the full patch. Would this require much work?

Måns Rullgård
[email protected]

2.6.0-test3-mm1 already contains O14.1

Other split patches can be found here:

http://kernel.kolivas.org/2.5

The split patches for >O14 on test3 vanilla do not work at the moment I'm afraid. It is much easier to maintain the patches against the one tree.

Con

2.6.0-test3-mm1 does already have O14.1, so you can apply O15 on top of what you have. Also, look at http://kernel.kolivas.org/2.5. There you will find patches against vanilla 2.6.

I'm getting some pretty bad starvation on 2.6.0-test3-mm1...

Let's say I'm compiling something on one of the consoles and then I decide to boot into KDE. Well, I haven't bothered to fix it yet, but xosview is in my session, and it doesn't work with the new kernel. So it just sits and hogs the CPU. That's not a real problem, but with this kernel it really does hog the CPU. At first, I have blocks of 5 seconds or so where I can't do anything (the mouse won't move and the screen won't redraw in X [and I can't get to a terminal during those busy times]), and then everything goes real smooth again. If I let xosview do whatever it's doing and don't kill it, the machine will become unresponsive in under one minute from launching xosview. At that point, I have to wait (I didn't time it precisely) a minute or more for the machine to show any signs of life. When the screen starts redrawing, it processes a few of my keystrokes, draws a little bit of the windows and then locks up again for another minute. If I'm lucky, I can get to a terminal to kill xosview, but the first time I just rebooted by hitting the power button. I have acpid setup to reboot when it gets the power button event. It seems compiling and then starting xosview makes this worse, it doesn't seem to be that bad unless I'm doing something else that's CPU-intensive. It still jerks around, but not bad and it seems to get better the longer xosview runs (if it can be called that ;-).

It's not just X that is slow either, if I manage to get back to the console I find that a running configure is at the same spot it was when xosview started. In fact, the stopping configures have reminded me several times that xosview had started so I could kill it before switching to vt7. Typing is equally difficult and I can expect to wait 30-45 seconds for even parts of my typing appear on the screen. Luckily, there are periods of time every minute or so when other processes get at the CPU...for 10-20 seconds, which happens to be just enough time to kill it from a console. But it takes a long time for an xterm start under these conditions....and I don't always make it out of X on the first try.

I haven't ever let it get to the point I needed to resort to the power button since, but it took three minutes before the computer even beeped indicating that the shutdown had started and another four before X finally closed. Even then by the time I finally got to the console all I got to see was the "killing <mumble>...", and then "rebooting now" (or whatever the kernel flashes before a reboot) messages.

This didn't happen on 2.6.0-test2-mm1, but...I compiled test3 with gcc 3.3.1, and the earlier kernels with 2.95.3. I don't know if that would cause this problem...

I'll try this patch and see if it makes any difference. If not, I probably should try compiling 2.6.0-test2-mm1 with 3.3.1 and see if that causes the same behavior. I'll let you know how it goes.

Earlier versions of your patch were smoother, but besides this problem, it's pretty good.

-Wes

You're seeing the priority inversion problem that remains that others have felt in wine applications. Don't waste your time with different gcc compilations. I'm working on the problem.

Con

Ok, finally had the chance to test O15.

Seems ok this side now with HT enabled. I did a few tests and here is just briefly some notes:

1) It does take a bit longer to compile anything when the system is under load. Roughly with two make -j{6,12}s going, with mm1 and about 1 second, and with bk3 and O15 about two seconds. I did not run too many passes on the bk3&O15 combination, so could be just some or other change in environment. Non issue for me.

2) XMMS/whatever still do not skip (nothing different from vanilla

    bk[13].

3) The only major difference now between vanilla bk, and bk3+O15 or mm[12] and O15, is that doing the 'window wiggle test' (*g*) do not start to jerk after about 10 seconds as in vanilla. It is either way of no significance to me, as I do not in general do anything that I can think of that simulate this. With vanilla though, response is there immediately if the window is let go of.

4) The evolution thread doing expunging (pop3 side) when finished getting mail do seem to take a bit longer. Guess it could be similar to the wine issues others have seen, but once again no real show stopper for me (I do expect some loss in response if there is high load).

5) On bk3+O15 it seems like load is a tad higher for the same type of tests (23-26 with vanilla, 23-28 with mm, and about 25-30 with vanilla bk3+o13). I guess it might be some changes between bk3 and mm that your stuff have a minor dependency on, or changes from bk1 to bk3, as bk1 was what I used for testing without O15.

All in all, mouse is fine, mozilla do not 'pause'/whatever when changing tabs/scrolling (both O15 and vanilla), XMMS do not skip, etc.

Regards,

Martin Schlemmer