On 08/09/2015 13:50, Parav Pandit wrote: > On Tue, Sep 8, 2015 at 2:06 PM, Haggai Eran <hagg...@mellanox.com> wrote: >> On 07/09/2015 23:38, Parav Pandit wrote: >>> +void devcgroup_rdma_uncharge_resource(struct ib_ucontext *ucontext, >>> + enum devcgroup_rdma_rt type, int num) >>> +{ >>> + struct dev_cgroup *dev_cg, *p; >>> + struct task_struct *ctx_task; >>> + >>> + if (!num) >>> + return; >>> + >>> + /* get cgroup of ib_ucontext it belong to, to uncharge >>> + * so that when its called from any worker tasks or any >>> + * other tasks to which this resource doesn't belong to, >>> + * it can be uncharged correctly. >>> + */ >>> + if (ucontext) >>> + ctx_task = get_pid_task(ucontext->tgid, PIDTYPE_PID); >>> + else >>> + ctx_task = current; >> So what happens if a process creates a ucontext, forks, and then the >> child creates and destroys a CQ? If I understand correctly, created >> resources are always charged to the current process (the child), but >> when it is destroyed the owner of the ucontext (the parent) will be >> uncharged. >> >> Since ucontexts are not meant to be used by multiple processes, I think >> it would be okay to always charge the owner process (the one that >> created the ucontext). > > I need to think about it. I would like to avoid keep per task resource > counters for two reasons. > For a while I thought that native fork() doesn't take care to share > the RDMA resources and all CQ, QP dmaable memory from PID namespace > perspective. > > 1. Because, it could well happen that process and its child process is > created in PID namespace_A, after which child is migrated to new PID > namespace_B. > after which parent from the namespace_A is terminated. I am not sure > how the ucontext ownership changes from parent to child process at > that point today. > I prefer to keep this complexity out if at all it exists as process > migration across namespaces is not a frequent event for which to > optimize the code for. > > 2. by having per task counter (as cost of memory some memory) allows > to avoid using atomic during charge(), uncharge(). > > The intent is to have per task (process and thread) to have their > resource counter instance, but I can see that its broken where its > charging parent process as of now without atomics. > As you said its ok to always charge the owner process, I have to relax > 2nd requirement and fallback to use atomics for charge(), uncharge() > or I have to get rid of ucontext from the uncharge() API which is > difficult due to fput() being in worker thread context. >
I think the cost of atomic operations here would normally be negligible compared to the cost of accessing the hardware to allocate or deallocate these resources. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/