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SubjectRe: [PATCH v16 00/13] support "task_isolation" mode
Hi Chris,

(CC Cavium people)

Thanks for your series.

On Fri, Nov 03, 2017 at 01:04:39PM -0400, Chris Metcalf wrote:
> Here, finally, is a new spin of the task isolation work (v16), with
> changes based on the issues that were raised at last year's Linux
> Plumbers Conference and in the email discussion that followed.
>
> This version of the patch series cleans up a number of areas that were
> a little dodgy in the previous patch series.
>
> - It no longer loops in the final code that prepares to return to
> userspace; instead, it sets things up in the prctl() and then
> validates when preparing to return to userspace, adjusting the
> syscall return value to -EAGAIN at that point if something doesn't
> line up quite correctly.
>
> - We no longer support the NOSIG mode that let you freely call into
> the kernel multiple times while in task isolation. This was always
> a little odd, since you really should be in sufficient control of
> task isolation code that you can explicitly stop isolation with a
> "prctl(PR_TASK_ISOLATION, 0)" before using the kernel for anything
> else. It also made the semantics of migrating the task confusing.
> More importantly, removing that support means that the only path
> that sets up task isolation is the return from prctl(), which allows
> us to make the simplification above.
>
> - We no longer try to signal the task isolation process from a remote
> core when we detect that we are about to violate its isolation.
> Instead, we just print a message there (and optionally dump stack),
> and rely on the eventual interrupt on the core itself to trigger the
> signal. We are always in a safe context to generate a signal when
> we enter the kernel, unlike when we are deep in a call stack sending
> an SMP IPI or whatever.
>
> - We notice the case of an unstable scheduler clock and return
> EINVAL rather than spinning forever with EAGAIN (suggestion from
> Francis Giraldeau).
>
> - The prctl() call requires zeros for arg3/4/5 (suggestion from
> Eugene Syromiatnikov).
>
> The kernel internal isolation API is also now cleaner, and I have
> included kerneldoc APIs for all the interfaces so that it should be
> easier to port it to additional architectures; in fact looking at
> include/linux/isolation.h is a good place to start understanding the
> overall patch set.
>
> I removed Catalin's Reviewed-by for arm64, and Christoph's Tested-by
> for x86, since this version is sufficiently different to merit
> re-review and re-testing.
>
> Note that this is not rebased on top of Frederic's recent housekeeping
> patch series, although it is largely orthogonal right now. After
> Frederic's patch series lands, task isolation is enabled with
> "isolcpus=nohz,domain,CPUS". We could add some shorthand for that
> ("isolcpus=full,CPUS"?) or just use it as-is.
>
> The previous (v15) patch series is here:
>
> https://lkml.kernel.org/r/1471382376-5443-1-git-send-email-cmetcalf@mellanox.com
>
> This patch series is available at:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile.git dataplane
>
> Some folks raised some good points at the LPC discussion and then in
> email discussions that followed. Rather than trying to respond to
> everyone in a flurry of emails, I'll answer some questions here:
>
>
> Why not just instrument user_exit() to raise the isolation-lost signal?
>
> Andy pointed me in this direction. The advantage is that you catch
> *everything*, by definition. There is a hook that can do this in the
> current patch set, but you have to #define DEBUG_TASK_ISOLATION
> manually to take advantage of it, because as written it has two issues:
>
> 1. You can't actually exit the kernel with prctl(PR_TASK_ISOLATION,0)
> because the user_exit hook kills you first.
> 2. You lose the ability to get much better diagnostics by waiting
> until you are further into kernel entry and know what you're doing.
>
> You could work around #2 in several ways, but #1 is harder. I looked
> at x86 for a while, and although you could imagine this, you really
> want to generate a lost-isolation signal on any syscall that isn't
> that exact prctl(), and it's awkward to try to do all of that checking
> before user_exit(). Since in any case we do want to have the more
> specific probes at the various kernel entry points where we generate
> the diagnostics, I felt like it wasn't the right approach to enable
> as a compilation-time default. I'm open to discussion on this though!
>
>
> Can't we do all the exit-to-userspace work with irqs disabled?
>
> In fact, it turns out that you can do lru_add_drain() with irqs
> disabled, so that's what we're doing in the patch series now.
>
> However, it doesn't seem possible to do the synchronous cancellation of
> the vmstat deferred work with irqs disabled, though if there's a way,
> it would be a little cleaner to do that; Christoph? We can certainly
> update the statistics with interrupts disabled via
> refresh_cpu_vm_stats(false), but that's not sufficient. For now, I
> just issue the cancellation during sys_prctl() call, and then if it
> isn't synchronized by the time we are exiting to userspace, we just
> jam in an EAGAIN and let userspace retry. In practice, this doesn't
> seem to ever happen.
>
>
> What about using a per-cpu flag to stop doing new deferred work?
>
> Andy also suggested we could structure the code to have the prctl()
> set a per-cpu flag to stop adding new future work (e.g. vmstat per-cpu
> data, or lru page cache). Then, we could flush those structures right
> from the sys_prctl() call, and when we were returning to user space,
> we'd be confident that there wasn't going to be any new work added.
>
> With the current set of things that we are disabling for task
> isolation, though, it didn't seem necessary. Quiescing the vmstat
> shepherd seems like it is generally pretty safe since we will likely
> be able to sync up the per-cpu cache and kill the deferred work with
> high probability, with no expectation that additional work will show
> up. And since we can flush the LRU page cache with interrupts
> disabled, that turns out not to be an issue either.
>
> I could imagine that if we have to deal with some new kind of deferred
> work, we might find the per-cpu flag becomes a good solution, but for
> now we don't have a good use case for that approach.
>
>
> How about stopping the dyn tick?
>
> Right now we try to stop it on return to userspace, but if we can't,
> we just return EAGAIN to userspace. In practice, what I see is that
> usually the tick stops immediately, but occasionally it doesn't; in
> this case I've always seen that nr_running is >1, presumably with some
> temporary kernel worker threads, and the user code just needs to call
> prctl() until those threads are done. We could structure things with
> a completion that we wait for, which is set by the timer code when it
> finally does stop the tick, but this may be overkill, particularly
> since we'll only be running this prctl() loop from userspace on cores
> where we have no other useful work that we're trying to run anyway.
>
>
> What about TLB flushing?
>
> We talked about this at Plumbers and some of the email discussion also
> was about TLB flushing. I haven't tried to add it to this patch set,
> because I really want to avoid scope creep; in any case, I think I
> managed to convince Andy that he was going to work on it himself. :)
> Paul McKenney already contributed some framework for such a patch, in
> commit b8c17e6664c4 ("rcu: Maintain special bits at bottom of
> ->dynticks counter").
>
> What about that d*mn 1 Hz clock?
>
> It's still there, so this code still requires some further work before
> it can actually get a process into long-term task isolation (without
> the obvious one-line kernel hack). Frederic suggested a while ago
> forcing updates on cpustats was required as the last gating factor; do
> we think that is still true? Christoph was working on this at one
> point - any progress from your point of view?

I tested your series on ThunderX 2 machine. When I run 10 giga-ticks test,
it always passed. If I run for more, the test exits like this:

# time ./isolation 1000
/sys devices: OK (using task isolation cpu 100)
prctl unaffinitized: OK
prctl on cpu 0: OK
==> hello, world
test_off: OK
Received signal 11 successfully
test_segv: OK
test_fault: OK
test_fault (SIGUSR1): OK
test_syscall: OK
test_syscall (SIGUSR1): OK
test_schedule: OK
test_schedule (SIGUSR1): OK
testing task isolation jitter for 1000000000000 ticks
ERROR: Program unexpectedly entered kernel.
INFO: loop times:
1 cycles (count: 128606844716)
2 cycles (count: 31558352292)
3 cycles (count: 4)
29 cycles (count: 437)
30 cycles (count: 1874)
31 cycles (count: 221)
57 cycles (count: 4)
58 cycles (count: 6)
59 cycles (count: 1)

real 15m58.643s
user 15m58.626s
sys 0m0.012s

I pass task_isolation_debug to boot parameters:
# cat /proc/cmdline
BOOT_IMAGE=/boot/Image-isol nohz_full=100-110 isolcpus=100-110 task_isolation_debug root=UUID=75b9dd5b-58d8-4a50-8868-004cb7c1f25f ro text

But dmesg looks empty...

I investigate it, but at now I have no ideas what happens. Have you seen
that before?

Anyway, we are going to include your test in our scenario, with some
modifications. I've added --dry-run option to make it easier to
demonstrate the effect of isolation on jitter. If you like it, feel
free to use this change.

Tested-by: Yury Norov <ynorov@caviumnetworks.com>

From 5c5823c1fc8441ab1486287679de77b8cea5154d Mon Sep 17 00:00:00 2001
From: Yury Norov <ynorov@caviumnetworks.com>
Date: Wed, 7 Mar 2018 02:41:08 +0300
Subject: [PATCH] isolation test: --dry-run mode

Add dry-run mode for better understanding the effect of isolation.
Also, make sanity checks on waitticks.

Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
---
tools/testing/selftests/task_isolation/isolation.c | 47 +++++++++++++++++-----
1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/task_isolation/isolation.c b/tools/testing/selftests/task_isolation/isolation.c
index 9c0b49619b40..e90a6c85fe2a 100644
--- a/tools/testing/selftests/task_isolation/isolation.c
+++ b/tools/testing/selftests/task_isolation/isolation.c
@@ -53,6 +53,7 @@
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
+#include <limits.h>
#include <assert.h>
#include <string.h>
#include <errno.h>
@@ -500,7 +501,7 @@ static void jitter_handler(int sig)
exit(exit_status);
}

-static void test_jitter(unsigned long waitticks)
+static void test_jitter(unsigned long waitticks, int flags)
{
u_int64_t start, last, elapsed;
int rc;
@@ -513,7 +514,8 @@ static void test_jitter(unsigned long waitticks)
rc = mlockall(MCL_CURRENT);
assert(rc == 0);

- set_task_isolation(PR_TASK_ISOLATION_ENABLE |
+ if (flags & PR_TASK_ISOLATION_ENABLE)
+ set_task_isolation(PR_TASK_ISOLATION_ENABLE |
PR_TASK_ISOLATION_SET_SIG(SIGUSR1));

last = start = get_cycle_count();
@@ -537,26 +539,49 @@ static void test_jitter(unsigned long waitticks)
} while (elapsed < waitticks);

jitter_test_complete = true;
- prctl_isolation(0);
+
+ if (flags & PR_TASK_ISOLATION_ENABLE)
+ prctl_isolation(0);
+
jitter_summarize();
}

int main(int argc, char **argv)
{
/* How many billion ticks to wait after running the other tests? */
- unsigned long waitticks;
+ long waitticks = 10;
+ int flags = PR_TASK_ISOLATION_ENABLE;
char buf[100];
char *result, *end;
FILE *f;

- if (argc == 1)
- waitticks = 10;
- else if (argc == 2)
- waitticks = strtol(argv[1], NULL, 10);
- else {
- printf("syntax: isolation [gigaticks]\n");
+ errno = 0;
+
+ switch (argc) {
+ case 1:
+ break;
+ case 2:
+ if (strcmp(argv[1], "--dry-run") == 0)
+ flags = 0;
+ else
+ waitticks = strtol(argv[1], NULL, 10);
+ break;
+ case 3:
+ if (strcmp(argv[1], "--dry-run") == 0)
+ flags = 0;
+
+ waitticks = strtol(argv[2], NULL, 10);
+ break;
+ default:
+ printf("syntax: isolation [--dry-run] [gigaticks]\n");
ksft_exit_fail();
}
+
+ if (errno || waitticks <= 0 || waitticks > (LONG_MAX / 1000000000)) {
+ printf("Gigaticks: bad number.\n");
+ ksft_exit_fail();
+ }
+
waitticks *= 1000000000;

/* Get a core from the /sys nohz_full device. */
@@ -637,7 +662,7 @@ int main(int argc, char **argv)
return exit_status;
}

- test_jitter(waitticks);
+ test_jitter(waitticks, flags);

return exit_status;
}
--
2.14.1
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