linux-kernel - Re: [PATCH v2 2/2] modules/kmod: replace implementation with a sempahore

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Message-ID: <20230407171929.xlxi7ewxxtrqo6z5@offworld>
Date:   Fri, 7 Apr 2023 10:19:29 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     david@...hat.com, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pmladek@...e.com,
        petr.pavlu@...e.com, prarit@...hat.com,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        rafael@...nel.org, christophe.leroy@...roup.eu, tglx@...utronix.de,
        peterz@...radead.org, song@...nel.org, rppt@...nel.org,
        willy@...radead.org, vbabka@...e.cz, mhocko@...e.com,
        dave.hansen@...ux.intel.com, colin.i.king@...il.com,
        jim.cromie@...il.com, catalin.marinas@....com, jbaron@...mai.com,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 2/2] modules/kmod: replace implementation with a
 sempahore

In the title: s/sempahore/semaphore

On Wed, 05 Apr 2023, Luis Chamberlain wrote:

>Simplfy the concurrency delimiter we user for kmod with the semaphore.
>I had used the kmod strategy to try to implement a similar concurrency
>delimiter for the kernel_read*() calls from the finit_module() path
>so to reduce vmalloc() memory pressure. That effort didn't provid yet
>conclusive results, but one thing that did became clear is we can use
>the suggested alternative solution with semaphores which Linus hinted
>at instead of using the atomic / wait strategy.
>
>I've stress tested this with kmod test 0008:
>
>time /data/linux-next/tools/testing/selftests/kmod/kmod.sh -t 0008
>
>And I get only a *slight* delay. That delay however is small, a few
>seconds for a full test loop run that runs 150 times, for about ~30-40
>seconds. The small delay is worth the simplfication IMHO.

Yes, code looks a lot nicer.

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>

>Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
>---
> kernel/module/kmod.c | 26 +++++++-------------------
> 1 file changed, 7 insertions(+), 19 deletions(-)
>
>diff --git a/kernel/module/kmod.c b/kernel/module/kmod.c
>index b717134ebe17..925eb85b8346 100644
>--- a/kernel/module/kmod.c
>+++ b/kernel/module/kmod.c
>@@ -40,8 +40,7 @@
>  * effect. Systems like these are very unlikely if modules are enabled.
>  */
> #define MAX_KMOD_CONCURRENT 50
>-static atomic_t kmod_concurrent_max = ATOMIC_INIT(MAX_KMOD_CONCURRENT);
>-static DECLARE_WAIT_QUEUE_HEAD(kmod_wq);
>+static DEFINE_SEMAPHORE(kmod_concurrent_max, MAX_KMOD_CONCURRENT);
>
> /*
>  * This is a restriction on having *all* MAX_KMOD_CONCURRENT threads
>@@ -148,29 +147,18 @@ int __request_module(bool wait, const char *fmt, ...)
>	if (ret)
>		return ret;
>
>-	if (atomic_dec_if_positive(&kmod_concurrent_max) < 0) {
>-		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
>-				    atomic_read(&kmod_concurrent_max),
>-				    MAX_KMOD_CONCURRENT, module_name);
>-		ret = wait_event_killable_timeout(kmod_wq,
>-						  atomic_dec_if_positive(&kmod_concurrent_max) >= 0,
>-						  MAX_KMOD_ALL_BUSY_TIMEOUT * HZ);
>-		if (!ret) {
>-			pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
>-					    module_name, MAX_KMOD_CONCURRENT, MAX_KMOD_ALL_BUSY_TIMEOUT);
>-			return -ETIME;
>-		} else if (ret == -ERESTARTSYS) {
>-			pr_warn_ratelimited("request_module: sigkill sent for modprobe %s, giving up", module_name);
>-			return ret;
>-		}
>+	ret = down_timeout(&kmod_concurrent_max, MAX_KMOD_ALL_BUSY_TIMEOUT);
>+	if (ret) {
>+		pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
>+				    module_name, MAX_KMOD_CONCURRENT, MAX_KMOD_ALL_BUSY_TIMEOUT);
>+		return ret;
>	}
>
>	trace_module_request(module_name, wait, _RET_IP_);
>
>	ret = call_modprobe(module_name, wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);
>
>-	atomic_inc(&kmod_concurrent_max);
>-	wake_up(&kmod_wq);
>+	up(&kmod_concurrent_max);
>
>	return ret;
> }
>--
>2.39.2
>