Path: csiph.com!eternal-september.org!feeder.eternal-september.org!aioe.org!bofh.it!news.nic.it!robomod From: Punit Agrawal Newsgroups: linux.kernel Subject: Re: [RFC v2 PATCH 2/7] KVM: Track the pid of the VM process Date: Tue, 06 Sep 2016 17:30:03 +0200 Message-ID: References: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: robomod@news.nic.it List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Approved: robomod@news.nic.it Lines: 119 Organization: linux.* mail to news gateway X-Original-Cc: kvm@vger.kernel.org, Marc Zyngier , Will Deacon , linux-kernel@vger.kernel.org, Steven Rostedt , Ingo Molnar , Paolo Bonzini , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-Original-Date: Tue, 06 Sep 2016 16:22:17 +0100 X-Original-Message-ID: <87poohhx3q.fsf@e105922-lin.cambridge.arm.com> X-Original-References: <1473093097-30932-1-git-send-email-punit.agrawal@arm.com> <1473093097-30932-3-git-send-email-punit.agrawal@arm.com> <20160906062252.GA30513@cbox> <87poohjqzk.fsf@e105922-lin.cambridge.arm.com> <20160906102542.GG30513@cbox> <877fapjng0.fsf@e105922-lin.cambridge.arm.com> <20160906112203.GO30513@cbox> X-Original-Sender: linux-kernel-owner@vger.kernel.org Xref: csiph.com linux.kernel:1477540 Christoffer Dall writes: > On Tue, Sep 06, 2016 at 12:07:59PM +0100, Punit Agrawal wrote: >> Christoffer Dall writes: >> >> > On Tue, Sep 06, 2016 at 10:51:27AM +0100, Punit Agrawal wrote: >> >> Hi Christoffer, >> >> >> >> Christoffer Dall writes: >> >> >> >> > On Mon, Sep 05, 2016 at 05:31:32PM +0100, Punit Agrawal wrote: >> >> >> Userspace tools such as perf can be used to profile individual >> >> >> processes. >> >> >> >> >> >> Track the PID of the virtual machine process to match profiling requests >> >> >> targeted at it. This can be used to take appropriate action to enable >> >> >> the requested profiling operations for the VMs of interest. >> >> >> >> >> >> Signed-off-by: Punit Agrawal >> >> >> Cc: Paolo Bonzini >> >> >> Cc: "Radim Krčmář" >> >> >> Cc: Christoffer Dall >> >> >> Cc: Marc Zyngier >> >> >> --- >> >> >> include/linux/kvm_host.h | 1 + >> >> >> virt/kvm/kvm_main.c | 2 ++ >> >> >> 2 files changed, 3 insertions(+) >> >> >> >> >> >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h >> >> >> index 9c28b4d..7c42c94 100644 >> >> >> --- a/include/linux/kvm_host.h >> >> >> +++ b/include/linux/kvm_host.h >> >> >> @@ -374,6 +374,7 @@ struct kvm_memslots { >> >> >> struct kvm { >> >> >> spinlock_t mmu_lock; >> >> >> struct mutex slots_lock; >> >> >> + struct pid *pid; >> >> >> struct mm_struct *mm; /* userspace tied to this vm */ >> >> >> struct kvm_memslots *memslots[KVM_ADDRESS_SPACE_NUM]; >> >> >> struct srcu_struct srcu; >> >> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c >> >> >> index 1950782..ab2535a 100644 >> >> >> --- a/virt/kvm/kvm_main.c >> >> >> +++ b/virt/kvm/kvm_main.c >> >> >> @@ -613,6 +613,7 @@ static struct kvm *kvm_create_vm(unsigned long type) >> >> >> spin_lock_init(&kvm->mmu_lock); >> >> >> atomic_inc(¤t->mm->mm_count); >> >> >> kvm->mm = current->mm; >> >> >> + kvm->pid = get_task_pid(current, PIDTYPE_PID); >> >> > >> >> > How dooes this deal with threading? Is the idea that the user by >> >> > specifying the main thread's pid will enable trapping for all vcpu >> >> > threads belonging to that VM? >> >> >> >> Yes that's correct - specifying the main thread PID will enable trapping >> >> for the VM (all vcpus). >> >> >> >> I am happy to move to a more suitable identifier if available. >> >> >> > >> > What is the 'main thread' ? >> > >> > Does something mandate that the VM is created by the thread group >> > leader? If not, is it not a bit strange from a user perspective, that >> > you have to find the specific subthread pid that created the vm to >> > enable this tracing for all vcpu threads and that the tgid doesn't work >> > in this case? >> >> Let me correct my terminology usage - the value recorded above (and used >> to identify the VM) should be the tgid. It is confusing because 'ps' >> reports it as pid. >> >> I picked the value as existing KVM code already uses the PID of the >> creating task (see kvm_create_vm_debugfs) to export VM statistics in >> debugfs. >> >> If I've got this wrong, then kvm_create_vm_debugfs also likely needs an >> update. >> >> What do you think? >> > When you do get_task_pid(current, PIDTYPE_PID) it actually gets the > kernel view of a PID which is the thead-id from userspace's point of > view, right? That makes sense. It seems to works here because the pid of the first task is also the tgid of the group. And I reckon it's the same assumption being made with debugfs code (more below). I've changed the first argument of the call to get_task_pid to current->group_leader. > > I don't see why this has to be the same as the debugfs code, as there it > makes potentially more sense to thread-specific, but for your case, are > you not targeting the behavior that a user can do "ps aux | grep qemu" > or whatever, and then set tracing for the reported PID (which is > actually a tgid)? The debugfs stats are not thread (vcpu) specific but for the VM. Both values, debugfs and here, are being used to represent the VM to the user. A mismatch in these identifiers will be very confusing. If you agree, I can separately send a patch to address this for VM debugfs directory as well. > > If this is indeed the case, then I don't think the current code supports > this if QEMU was ever changed to create the VM with a different thread > than the tgl. > > That being said, I'm typically wrong when I talk about userspace. > > -Christoffer > _______________________________________________ > kvmarm mailing list > kvmarm@lists.cs.columbia.edu > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm