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Fri, 2 Apr 2021 16:44:33 +0000 (GMT) From: Nathan Lynch To: Laurent Dufour Subject: Re: [PATCH v2] pseries: prevent free CPU ids to be reused on another node In-Reply-To: References: <20210325093512.57856-1-ldufour@linux.ibm.com> <87a6qgbyk6.fsf@linux.ibm.com> Date: Fri, 02 Apr 2021 11:44:32 -0500 Message-ID: <877dlkbpqn.fsf@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 X-Proofpoint-GUID: HJD8wR6yREK7Ai04wfNu96h7_W125uRM X-Proofpoint-ORIG-GUID: HJD8wR6yREK7Ai04wfNu96h7_W125uRM X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369, 18.0.761 definitions=2021-04-02_09:2021-04-01, 2021-04-02 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 priorityscore=1501 mlxscore=0 phishscore=0 spamscore=0 clxscore=1015 adultscore=0 suspectscore=0 malwarescore=0 lowpriorityscore=0 impostorscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2103310000 definitions=main-2104020116 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Srikar Dronamraju , cheloha@linux.ibm.com, linux-kernel@vger.kernel.org, paulus@samba.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Laurent Dufour writes: > Le 02/04/2021 =C3=A0 15:34, Nathan Lynch a =C3=A9crit=C2=A0: >> Laurent Dufour writes: >>> When a CPU is hot added, the CPU ids are taken from the available mask = from >>> the lower possible set. If that set of values was previously used for C= PU >>> attached to a different node, this seems to application like if these C= PUs >>> have migrated from a node to another one which is not expected in real >>> life. >>=20 >> This seems like a problem that could affect other architectures or >> platforms? I guess as long as arch code is responsible for placing new >> CPUs in cpu_present_mask, that code will have the responsibility of >> ensuring CPU IDs' NUMA assignments remain stable. > > Actually, x86 is already handling this issue in the arch code specific > code, see 8f54969dc8d6 ("x86/acpi: Introduce persistent storage for > cpuid <-> apicid mapping"). I didn't check for other architectures but > as CPU id allocation is in the arch part, I believe this is up to each > arch to deal with this issue. > > Making the CPU id allocation common to all arch is outside the scope > of this patch. Well... we'd better avoid a situation where architectures impose different policies in this area. (I guess we're already in this situation, which is the problem.) A more maintainable way to achieve that would be to put the higher-level policy in arch-independent code, as much as possible. I don't insist, though. >> I don't know, should we not fail the request instead of doing the >> ABI-breaking thing the code in this change is trying to prevent? I >> don't think a warning in the kernel log is going to help any >> application that would be affected by this. > > That's a really good question. One should argue that the most > important is to satisfy the CPU add operation, assuming that only few > are interested in the CPU numbering, while others would prefer the CPU > adding to fail (which may prevent adding CPUs on another nodes if the > whole operation is aborted as soon as a CPU add is failing). > > I was conservative here, but if failing the operation is the best > option, then this will make that code simpler, removing the backward > jump. > > Who is deciding? I favor failing the request. Apart from the implications for user space, it's not clear to me that allowing the cpu-node relationship to change once initialized is benign in terms of internal kernel assumptions (e.g. sched domains, workqueues?). And as you say, it would make for more straightforward code.