From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4BBD82135AD for ; Sat, 22 Nov 2025 00:44:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763772244; cv=none; b=F2ibi3r+7+gY/+Ri+PebH5Vm3nSICdzgMBL7FC9rGav5PKAgC1FNDVLpW0nz15ehx1zpLwgBp9LXlo97QvxXttG2j2aN3zQp/k1moGlAfTN4o0I3aG0f53HQAsxVQzwvHIM7gwZo9lPhad3Xw7bYPmXNR94KZMowBn4ObDRq8+4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763772244; c=relaxed/simple; bh=6DHdaoKx0Ho3b474XdbIkzMlcsvyB5LyLdhqnAoGVHU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=MeCH+7oNFLukMogTXaSDfKKOm95+9hvvk8QrG2q+EpotU+fiEHKzpd+DE/zn0YLMBN5qTOs96hfiFezclzO4oxigOx7Hd/HhTcN+7/yIY0FEnoeupGaHljw0hJArrTale8py4GVEIHkc+8liA531MuuVUrqZd2+BdoFe2uoO0UM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=O5bWrSMy; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="O5bWrSMy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1023FC4CEFB; Sat, 22 Nov 2025 00:44:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763772243; bh=6DHdaoKx0Ho3b474XdbIkzMlcsvyB5LyLdhqnAoGVHU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=O5bWrSMyKZ7g8S0KxqjpIcL1YlYwjtSyUkGBlBX3pIPnq+mdV97nC2kzgWtpfW/xj DAJEYZS0ek/wc9slll1mDZQ3Z6azMrhVZLB80OaScHcfocNy949JZYLhVbtevMeEMq BvEokLZFjkSTqA5A2g9hrq8Le7bQO+RhSRDhUbBzFfsu225Bfuby3vaWIN092wJvp4 Kn2fHkCfmFZ/yh5XYpl2Qfrt1QaWkQhDuRTTHoY4BbOG4FjGCDmzqhZGgno8DFw2Vv s+cMygv8f+Kpew2OA7YKepA74BigMpejNbKYaziLK7nh0wOu0Pek2AzubuNrlkEo1f 26Jl1+w9RgaIA== Date: Fri, 21 Nov 2025 17:43:58 -0700 From: Nathan Chancellor To: Thomas Gleixner Cc: LKML , Peter Zijlstra , Gabriele Monaco , Mathieu Desnoyers , Michael Jeanson , Jens Axboe , "Paul E. McKenney" , "Gautham R. Shenoy" , Florian Weimer , Tim Chen , Yury Norov , Shrikanth Hegde Subject: Re: [patch V5 20/20] sched/mmcid: Switch over to the new mechanism Message-ID: <20251122004358.GB2682494@ax162> References: <20251119171016.815482037@linutronix.de> <20251119172550.280380631@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251119172550.280380631@linutronix.de> Hi Thomas, On Wed, Nov 19, 2025 at 06:27:22PM +0100, Thomas Gleixner wrote: > Now that all pieces are in place, change the implementations of > sched_mm_cid_fork() and sched_mm_cid_exit() to adhere to the new strict > ownership scheme and switch context_switch() over to use the new > mm_cid_schedin() functionality. > > The common case is that there is no mode change required, which makes > fork() and exit() just update the user count and the constraints. > > In case that a new user would exceed the CID space limit the fork() context > handles the transition to per CPU mode with mm::mm_cid::mutex held. exit() > handles the transition back to per task mode when the user count drops > below the switch back threshold. fork() might also be forced to handle a > deferred switch back to per task mode, when a affinity change increased the > number of allowed CPUs enough. > > Signed-off-by: Thomas Gleixner > Reviewed-by: Mathieu Desnoyers Our CI started seeing a hang in QEMU after getting to userspace when booting with a single CPU (our default logic when using TCG instead of KVM) that I bisected to this change as commit 2635fb0f0973 ("sched/mmcid: Switch over to the new mechanism") in -next. https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/19557323049/job/56003155890 $ make -skj"$(nproc)" ARCH=x86_64 CROSS_COMPILE=x86_64-linux- clean defconfig bzImage $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/x86_64-rootfs.cpio.zst | zstd -d >rootfs.cpio $ qemu-system-x86_64 \ -display none \ -nodefaults \ -M q35 \ -d unimp,guest_errors \ -append 'console=ttyS0 earlycon=uart8250,io,0x3f8' \ -kernel arch/x86/boot/bzImage \ -initrd rootfs.cpio \ -cpu host \ -enable-kvm \ -m 512m \ -smp 1 \ -serial mon:stdio [ 0.000000] Linux version 6.18.0-rc4-00066-g2635fb0f0973 (nathan@ax162) (x86_64-linux-gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.45) #1 SMP PREEMPT_DYNAMIC Fri Nov 21 17:30:21 MST 2025 ... [ 0.946720] Freeing unused kernel image (initmem) memory: 2812K [ 0.947864] Write protecting the kernel read-only data: 28672k [ 0.949257] Freeing unused kernel image (text/rodata gap) memory: 1540K [ 0.950579] Freeing unused kernel image (rodata/data gap) memory: 760K [ 0.991764] x86/mm: Checked W+X mappings: passed, no W+X pages found. [ 0.993000] Run /init as init process If I change '-smp 1' to '-smp 2', userspace runs properly. At the parent change, this issue does not exist but it is obviously possible that this change exposes a bug from earlier in the series, I did not test. Cheers, Nathan # bad: [d724c6f85e80a23ed46b7ebc6e38b527c09d64f5] Add linux-next specific files for 20251121 # good: [fd95357fd8c6778ac7dea6c57a19b8b182b6e91f] Merge tag 'sched_ext-for-6.18-rc6-fixes-2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext git bisect start 'd724c6f85e80a23ed46b7ebc6e38b527c09d64f5' 'fd95357fd8c6778ac7dea6c57a19b8b182b6e91f' # good: [d8d0238c7ea639aa8c9f8c76d089e684e283ea87] Merge branch 'libcrypto-next' of https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git git bisect good d8d0238c7ea639aa8c9f8c76d089e684e283ea87 # good: [84abc3686f608efd4914a5b4d651307e32ea4000] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git git bisect good 84abc3686f608efd4914a5b4d651307e32ea4000 # bad: [c709a6758d17f1f2bccadb18cc82bccecdf216ff] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git git bisect bad c709a6758d17f1f2bccadb18cc82bccecdf216ff # skip: [8d00858e9326dafe5e27072e3cee5c23b60e7281] Merge branch 'master' of https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git git bisect skip 8d00858e9326dafe5e27072e3cee5c23b60e7281 # good: [8be00d1ba3a1305469f6747d0285c4496a6855ee] KVM: arm64: GICv3: Completely disable trapping on vcpu exit git bisect good 8be00d1ba3a1305469f6747d0285c4496a6855ee # good: [4daaead949b331c271f6df4162f39589088fe27b] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git git bisect good 4daaead949b331c271f6df4162f39589088fe27b # bad: [b1a2fa6d34e27225c2b3f79840f198227af1f53d] Merge branch into tip/master: 'x86/sev' git bisect bad b1a2fa6d34e27225c2b3f79840f198227af1f53d # bad: [468f14a4c5c9def982ae9a74344fa6f905a9b725] Merge branch into tip/master: 'locking/core' git bisect bad 468f14a4c5c9def982ae9a74344fa6f905a9b725 # bad: [705d7cad382be9b3b52b89b220e77b17cf24322a] Merge branch 'sched/core' into core/rseq, to resolve conflict git bisect bad 705d7cad382be9b3b52b89b220e77b17cf24322a # good: [472931e757fb3dfad1f78ce6f5abd821155433b2] sched/mmcid: Convert mm CID mask to a bitmap git bisect good 472931e757fb3dfad1f78ce6f5abd821155433b2 # good: [7f829bde94b1c97b1804fa5860e066ea49dbfca3] sched/core: Optimize core cookie matching check git bisect good 7f829bde94b1c97b1804fa5860e066ea49dbfca3 # good: [d206fbad9328ddb68ebabd7cf7413392acd38081] sched/fair: Revert max_newidle_lb_cost bump git bisect good d206fbad9328ddb68ebabd7cf7413392acd38081 # good: [340af997d25dab0f05c4de8399d656b112592a93] sched/mmcid: Provide CID ownership mode fixup functions git bisect good 340af997d25dab0f05c4de8399d656b112592a93 # bad: [2635fb0f0973c57c45f03708d52e827ec99ac78e] sched/mmcid: Switch over to the new mechanism git bisect bad 2635fb0f0973c57c45f03708d52e827ec99ac78e # good: [2644779ec144d3e8cce5fed9623b47e70b3e0422] irqwork: Move data struct to a types header git bisect good 2644779ec144d3e8cce5fed9623b47e70b3e0422 # good: [cba5e581161e379037a94f5a75d1a61bd1ccce3b] sched/mmcid: Implement deferred mode change git bisect good cba5e581161e379037a94f5a75d1a61bd1ccce3b # first bad commit: [2635fb0f0973c57c45f03708d52e827ec99ac78e] sched/mmcid: Switch over to the new mechanism