With the growing complexity of smartphone systems, effective tracing becomes vital for enhancing their stability and optimizing the user experience. Unfortunately, existing tracing tools are inefficient in smartphone scenarios. Their distributed designs (with either per-core or per-thread buffers) prioritize performance but lead to missing crucial clues with high probability. While these problems can be overlooked in previous scenarios (e.g., servers), they drastically limit the usefulness of tracing on smartphones.To enable efficient tracing on smartphones, we propose BTrace: a tracing tool that combines the performance benefits of per-core buffers with the capability of retaining longer continuous traces by partitioning a global buffer into multiple blocks, which are dynamically assigned to the most demanding cores.BTrace further gracefully handles unique requirements of modern smartphones, e.g., core oversubscription and resizing. BTrace has been deployed in production, recording an average of 2x continuous traces compared to the current best-performing tracer (Linux ftrace) and improving performance by 20%. Using BTrace, we successfully identified numerous bugs that require traces of long duration and are challenging to locate with existing tracers.