Re: [PATCH] binder_alloc: Move alloc_page() out of mmap_rwsem to reduce the lock duration

[Hillf Danton <hdanton@sina.com>]

On Tue, Sep 03, 2024 at 10:50:09AM +1200, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> The mmap_write_lock() can block all access to the VMAs, for example page
> faults. Performing memory allocation while holding this lock may trigger
> direct reclamation, leading to others being queued in the rwsem for an
> extended period.
> We've observed that the allocation can sometimes take more than 300ms,
> significantly blocking other threads. The user interface sometimes
> becomes less responsive as a result. To prevent this, let's move the
> allocation outside of the write lock.

I suspect concurrent allocators make things better wrt response, cutting
alloc latency down to 10ms for instance in your scenario. Feel free to
show figures given Tangquan's 48-hour profiling.

> A potential side effect could be an extra alloc_page() for the second
> thread executing binder_install_single_page() while the first thread
> has done it earlier. However, according to Tangquan's 48-hour profiling
> using monkey, the likelihood of this occurring is minimal, with a ratio
> of only 1 in 2400. Compared to the significantly costly rwsem, this is
> negligible.
> On the other hand, holding a write lock without making any VMA
> modifications appears questionable and likely incorrect. While this
> patch focuses on reducing the lock duration, future updates may aim
> to eliminate the write lock entirely.

If spin, better not before taking a look at vm_insert_page().