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Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: htmxD1hD3Bxj2r95DCQdn/yjzyVKJ2tTwrAykneuAiOdLq4MhaprAG9fvQ/CR/NicUSvFE7gpzLBWObsQ2Qoj3KGdJdfQNG7ie/07LNEnoI= X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN9PR03MB6171 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/09/2023 4:04 pm, Peter Zijlstra wrote: > On Sun, Sep 10, 2023 at 11:32:32AM -0700, Linus Torvalds wrote: > >> I was hoping that we'd have some generic way to deal with this where >> we could just say "this thing is reschedulable", and get rid of - or >> at least not increasingly add to - the cond_resched() mess. > Isn't that called PREEMPT=y ? That tracks precisely all the constraints > required to know when/if we can preempt. > > The whole voluntary preempt model is basically the traditional > co-operative preemption model and that fully relies on manual yields. > > The problem with the REP prefix (and Xen hypercalls) is that > they're long running instructions and it becomes fundamentally > impossible to put a cond_resched() in. Any VMM - Xen isn't special here. And if we're talking about instructions, then CPUID, GETSEC and ENCL{S,U} and plenty of {RD,WR}MSRs in in a similar category, being effectively blocking RPC operations to something else in the platform. The Xen evtchn upcall logic in Linux does cond_resched() when possible.  i.e. long-running hypercalls issued with interrupts enabled can reschedule if an interrupt occurs, which is pretty close to how REP works too. ~Andrew