On Wed, Mar 07, 2018 at 05:39:09PM +0000, Suzuki K Poulose wrote: > On 09/02/18 18:58, Dave Martin wrote: > >On Fri, Feb 09, 2018 at 05:55:12PM +0000, Suzuki K Poulose wrote: > >>We enable hardware DBM bit in a capable CPU, very early in the > >>boot via __cpu_setup. This doesn't give us a flexibility of > >>optionally disable the feature, as the clearing the bit > >>is a bit costly as the TLB can cache the settings. Instead, > >>we delay enabling the feature until the CPU is brought up > >>into the kernel. We use the feature capability mechanism > >>to handle it. > >> > >>The hardware DBM is a non-conflicting feature. i.e, the kernel > >>can safely run with a mix of CPUs with some using the feature > >>and the others don't. So, it is safe for a late CPU to have > >>this capability and enable it, even if the active CPUs don't. > >> > >>To get this handled properly by the infrastructure, we > >>unconditionally set the capability and only enable it > >>on CPUs which really have the feature. Also, we print the > >>feature detection from the "matches" call back to make sure > >>we don't mislead the user when none of the CPUs could use the > >>feature. > >> > >>Cc: Catalin Marinas <[email protected]> > >>Cc: Dave Martin <[email protected]> > >>Signed-off-by: Suzuki K Poulose <[email protected]> > >>--- > >>Changes since V2 > >> - Print the feature detection message only when at least one CPU > >> is actually using it. > > > >>+static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap, > >>+ int __unused) > >>+{ > >>+ static bool detected = false; > >>+ /* > >>+ * DBM is a non-conflicting feature. i.e, the kernel can safely > >>+ * run a mix of CPUs with and without the feature. So, we > >>+ * unconditionally enable the capability to allow any late CPU > >>+ * to use the feature. We only enable the control bits on the > >>+ * CPU, if it actually supports. > >>+ * > >>+ * We have to make sure we print the "feature" detection only > >>+ * when at least one CPU actually uses it. So check if this CPU > >>+ * can actually use it and print the message exactly once. > >>+ * > >>+ * This is safe as all CPUs (including secondary CPUs - due to the > >>+ * LOCAL_CPU scope - and the hotplugged CPUs - via verification) > >>+ * goes through the "matches" check exactly once. Also if a CPU > >>+ * matches the criteria, it is guaranteed that the CPU will turn > >>+ * the DBM on, as the capability is unconditionally enabled. > >>+ */ > >>+ if (!detected && cpu_can_use_dbm(cap)) { > >>+ detected = true; > >>+ pr_info("detected feature: Hardware dirty bit management\n"); > >>+ } > > > >Can we just do > > > > if (cpu_can_use_dbm(cap)) > > pr_info_once(...); > > > >Then we can get rid of "detected". > > The reason for open coding is the cost of cpu_can_use_dbm() with > addition of black listed CPUs in the next patch in the series.
Oh, I see. Yes, that makes sense. There's obvious raciness here, but I guess pr_info_once() doesn't defend against that either. In practice, we don't race booting of two CPUs against each other IIUC. If you really like you could make detected __read_mostly like printk_once() does, but that's no big deal if this is not on a hot path (and probably not even then). Cheers ---Dave
