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linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at, mjguzik@gmail.com Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible In-reply-to: Message-ID: <877cmqd066.fsf@oracle.com> Date: Thu, 09 Nov 2023 14:36:17 -0800 Content-Type: text/plain 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X-MS-Exchange-CrossTenant-AuthSource: CO6PR10MB5409.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Nov 2023 22:36:18.5542 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: i4J4fhh7iXNKPV/BscxLkVIkfCr4hFbvzIz1gsmXPjWnYONFV0tl7jd3AUaS9KWhuJpw+vvf50I8iBzmP7Fu0PWjWMrlHsg4CdtuYOFOMwU= X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR10MB6788 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.987,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2023-11-09_15,2023-11-09_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=386 phishscore=0 spamscore=0 suspectscore=0 adultscore=0 bulkscore=0 malwarescore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311060000 definitions=main-2311090164 X-Proofpoint-GUID: 225g_zMv69bIZ7a4hh_W6NHeSUAbxeYz X-Proofpoint-ORIG-GUID: 225g_zMv69bIZ7a4hh_W6NHeSUAbxeYz Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mark Rutland writes: > On Wed, Nov 08, 2023 at 04:34:41PM -0800, Ankur Arora wrote: >> Mark Rutland writes: >> >> > On Tue, Nov 07, 2023 at 01:56:46PM -0800, Ankur Arora wrote: >> >> What's broken: >> >> - ARCH_NO_PREEMPT (See patch-45 "preempt: ARCH_NO_PREEMPT only preempts >> >> lazily") >> >> - Non-x86 architectures. It's trivial to support other archs (only need >> >> to add TIF_NEED_RESCHED_LAZY) but wanted to hold off until I got some >> >> comments on the series. >> >> (From some testing on arm64, didn't find any surprises.) >> > >> > When you say "testing on arm64, didn't find any surprises", I assume you mean >> > with an additional patch adding TIF_NEED_RESCHED_LAZY? >> >> Yeah. And, handling that in the user exit path. >> >> > Note that since arm64 doesn't use the generic entry code, that also requires >> > changes to arm64_preempt_schedule_irq() in arch/arm64/kernel/entry-common.c, to >> > handle TIF_NEED_RESCHED_LAZY. >> >> So, the intent (which got muddied due to this overly large series) >> was to delay handling TIF_NEED_RESCHED_LAZY until we are about to >> return to user. > > Ah, I missed that detail -- thanks for clarifying! > >> I think arm64_preempt_schedule_irq() should only handle TIF_NEED_RESCHED >> and the _TIF_NEED_RESCHED_LAZY should be handled via _TIF_WORK_MASK >> and do_notify_resume(). > > Digging a bit more, I think that should still work. > > One slight clarification: arm64_preempt_schedule_irq() doesn't look at > TIF_NEED_RESCHED today, as it relies on the scheduler IPI calling > preempt_fold_need_resched() to propogate TIF_NEED_RESCHED into > PREEMPT_NEED_RESCHED. That should still work since this series makes > preempt_fold_need_resched() check tif_need_resched(RESCHED_eager). > > I was a bit cnofused because in the generic entry code, > irqentry_exit_cond_resched() explicitly checks for TIF_NEED_RESCHED, and I'm > not sure why it does that rather than relying on the scheduler IPI as above. Yeah I found that confusing as well. I suspect the reason is that not all archs do the folding and we need the explicit check for those that don't. -- ankur